A - Z Guide of Gemstones

Agate is a type of quartz, which occurs in nodular masses in rocks such as volcanic lavas. When split open, they reveal an amazing variety of colours and patterns that distinguish it from others kinds of Chalcedony.

As a porous stone, agate is often dyed or stained to enhance its natural colour. There are, of course, many varieties of agate including Moss Agate also known as Mocha Stone, Landscape Agate and Fire Agate, to name but a few.

Mohs Scale of Hardness: 7

Sources: Idar-Oderstein (Germany) – most famous area for agate, Uraguay and Brazil

The most popular purple variety of Quartz is the Amethyst gemstone. Its name derives from the Greek word ‘amethystos’, which translates as ‘not drunken’, as Amethyst was thought to ward off drunkenness when worn.

In ancient times, Amethyst was highly regarded among precious gemstones such as Ruby and Emerald but discoveries of huge Amethyst deposits since the 1800s have made the gem fairly inexpensive and very obtainable. Amethyst colours range from light to dark purple and the transparent deep purple colours are the most highly regarded.

Amethyst is dichroic, showing a bluish or reddish purple tinge when viewed from different angles. Its colour is most often caused by iron impurities, though it can also be coloured by natural radiation exposure. Amethyst is sometimes heat treated to deepen the colour, or to change the colour to yellow to form Citrine.

Mohs Scale of Hardness: 7

Sources: Brazil is the largest producer of Amethyst. It is found there in the states of Minas Gerais, Rio Grande do Sul, Maraba and Bahia.
Other important deposits are in Uruguay (in Artigas), Argentina, Bolivia, Mexico, Namibia, Zambia, South Africa, Madagascar, Canada (Ontario), and the United States (Arizona, North Carolina, Georgia, and Maine).

Birthstone: February

See also Ametrine

Andesine is a relatively new gem on the market, its lovely reddish-pink-orange colour rivalling that of other more valuable gemstones of similar appearance. Andesine belongs to the same plagioclase feldspar family as Labradorite and shares a very similar chemical composition.

Indeed, there is a real debate at present as to the difference between red Labradorite and red Andesine. The gemstone is often referred to as ‘Andesine-Labradorite’ for this reason. To add to the controversy, it has been discovered that much red ‘Andesine’ has been artificially produced by subjecting Labradorite to diffusion treatment with copper.

Mohs Scale of Hardness: 6

Sources:  Congo, China, Mongolia, or Tibet, but its actual origin, or whether it has been diffusion treated is rarely disclosed.

Also see Labradorite

Apatite is a phosphate mineral, more commonly known for its use in the manufacturer of fertilizer than as a gemstone due to its biological properties. Hydroxlapatite, for example, is an important component of the bones and teeth of most animals, including humans.

Apatite is normally not faceted due to its hardness of only 5 on Mohs scale. However, when cut correctly, the stones are bright with strong colours, and come in a variety of transparent and opaque forms.

Blue Burmese Apatite is strongly diochroic (two colours), showing colourless or blue when viewed from different directions. Fibrous blue gem material from Burma and Sri Lanka may be cut as cabochons to show a cat’s-eye feature.

Fun fact: Apatite is derived from the Greek word apate, meaning ‘deceit’. This is fitting since Apatite is similar in appearance to so many minerals.

Colours: Colorless, white, yellow, brown, grey, red, pink, purple, blue, green

Mohs Scale of Hardness: 5

Sources:  Brazil, Burma, Sri Lanka, Kola Peninsula (Russia), Canada, East Africa, Sweden, Spain and Mexico

The incredible popularity of Aquamarine is testament to its beauty. While darker stones command higher prices, the paler shades are equally beautiful and lend a certain delicacy to the stone that make it incredibly wearable.

According to legend, Aquamarine was discovered in the treasure chest of a school of mermaids and has, since ancient times, been regarded as a stone of good luck for sailors. Unlike its cousin, Emerald, Aquamarine is much more commonly free of inclusions and is far less brittle.

Mohs Scale of Hardness: 7

Sources: Mozambique, Afganistan, Brazil

Birthstone: March

Beryllium-aluminium-silicate or Beryl is widely used as a gem material. In its pure form (also known as Goshenite), beryl is colourless. However, stored foreign substances give it different colours and different names. 

The best known and most popular type of beryl is Aquamarine, which is coloured sea-blue by the presence of iron. In turn, the presence of manganese turns beryl into a pretty, feminine pink variety called Morganite.

Other types of beryl include the much admired Emerald and Heliodor.

Mohs Scale of Hardness: 7.5

Sources: South America, Madagascar, Ukraine. Also see individual gemstones.

Bloodstone, also known as Heliotrope, can come in a variety of forms including opaque, green, and spotted. It is normally used for decorative carvings and cameos.

Very dark green bloodstone is spotted with red due to the presence of iron oxides. It is these distinctive red spots which resemble blood, giving the stone its name. In the Middle Ages, bloodstone was attributed with special powers, as the spots were thought to be the blood of Jesus Christ.

Mohs Scale of Hardness: 7

Sources: India, Brazil, China, Australia and USA

Citrine is the yellow, orange, or reddish-brown variety of Quartz. It is usually obtained by heat-treatment of Amethyst or Smokey Quartz. It takes a relatively low temperature to change the colour light to golden yellow, and heating to higher temperatures will give the stone a darker yellow to brownish-red colour. The name Citrine is derived from the citron fruit, a yellow fruit similar to the lemon.

Citrine has been used to imitate Topaz, and was once referred to as Brazilian Topaz.

Mohs Scale of Hardness: 7

Sources: Brazil, Spain and Madagascar

The word ‘diamond’ comes from the Greek word ‘adámas’, which means ‘unbreakable’. As the hardest mineral on earth in combination with its exceptional lustre and brilliant fire, Diamond has become the most highly prized of all gems. Pure, colourless Diamond is the most popular, but other varieties (from yellow, brown, and green through to pink, blue, grey and even black) are also found, depending on the impurities present.

No gemstone is said to generate as much allure and interest as Diamond, and is the most popular choice for engagement rings.

Roughly twenty percent of Diamonds mined are used in jewellery, as most rough material is unsuitable for gemstone use. The vast majority is either opaque or too heavily flawed to be of gem quality. A flawless Diamond is exceptional, as most Diamonds contain flaws to some extent, even if such flaws are not always obvious.

The immense hardness of a Diamond contributes to its suitability as a gemstone – the only thing that can scratch it is another Diamond. This resistance lends it the ability to withstand daily wear and tear beyond the capabilities of most gemstones. Diamonds are graded by colour, cut, clarity and carat (weight) – The Four Cs.

Many stories surround the gift of a Diamond as a token of affection.  The ancient Greeks believed that the fire of a Diamond reflected the flame of everlasting love.  Archduke Maximilian of Austria apparently agreed, as in 1477 he gave a Diamond ring to his fiancée, Mary of Burgundy.  And so began the tradition of presenting the Diamond as a promise of marriage, making it the most desirable and popular choice of gemstone for engagement rings.  Use our Diamond Guide, to help you choose the perfect diamond.

Mohs Scale of Hardness: 10

Sources: Australia, Ghana, Sierra Leone, Zaire, USSR, USA and Brazil

Fine Emeralds, of rich colour and completely devoid of inclusions are incredibly rare, and even more costly than Diamonds. However, flawless or not, Emeralds are unquestionably beautiful and have been treasured throughout history, the earliest mines having been found near the Red Sea in Egypt at around 3000 BC.

Today, rich green Columbian Emeralds re particularly sought-after and fetch high prices. Emeralds are one of the few gemstones to have had a cut created and named after them. The ‘emerald cut’, with its rectangular or square shape and bevelled edges, displays the Emerald at its best, while also minimising the risk of potential fracture. The cleanness of the emerald cut has made it a popular choice for many other gemstones and sets off stones particularly well in dress rings or simple pendants.

Mohs Scale of Hardness: 7.5

Sources: Colombia, Austria, India, Australia, Brazil, South Africa, Egypt, the USA, Norway, Pakistan and Zimbabwe

Also see Beryl

Birthstone: May

The colour most commonly associated with Garnet is red. The deep red of Bohemian Garnet was hugely popular in the late 18th  and 19th Centuries and examples of these stones cut small and en cabochon, and clustered together on brooches, hair pins and rings can be found in many antique shops and markets. Garnet, however, has many faces and can be green (Tsavorite and Demantoid), orange (Mandarin) and purple (colour change Garnets).

PYROPE
Pyrope Garnets (also known as Bohemian Garnets due to their copious mining in the former kingdom of Bohemia) are blood red in colour and often resemble Ruby. This fiery stone is often found inclusion free.

ALMANDINE
Almandine Garnets can be found in larger sizes than Pyropes. They are, in general, a darker red than Pyrope and rarely transparent.

RHODOLITE
Rhodolite Garnet is a mixed crystal of Pyrope and Almandine. This variety of Garnet is hugely popular and has a fantastic raspberry red undertone.

SPESSARTITES
Until recently, the rarity of Spessartites had meant that they were mainly the reserve of collectors and gem specialists. The discovery of a large deposit in Namibia changed things, however. When this deposit was exhausted, another discovery in Nigeria meant that Spessarite Garnets became much more available on the market.

Mohs Scale of Hardness: 7.5

Sources: Worldwide

Birthstone: January

Heliodor is the yellow or golden yellow form of Beryl. The golden colour is due to the presence of iron in place of aluminium in the structure of the crystal.

Heliodor has always been linked with the sun due to its name being derived from the Greek word meaning ‘Gift of the Sun’.

Mohs Scale of Hardness: 7.5

Sources: The best quality stones are from the Urals (Russia). Brazillian Helidor is often a pale yellow and step-cut to give depth of colour.

Also see Beryl

Labradorite is the plagioclase feldspar that is most commonly faceted as a gem. It comes in orange, yellow, colourless and red but the material that shows a play of colour, known as ‘schiller’, is the most popular for use in jewellery.

Mohs Scale of Hardness: 6

Sources: Labrador (Canada), Finland and Norway

Also see Spectrolite

The pink, rose, peach and violet varieties of Beryl are known under the name Morganite for their presence of manganese and are named after the American banker and gem enthusiast J. Pierpoint Morgan.

Morganite is dichroic, which means it shows either two shades of the body colour or one shade and colourless.

Mohs Scale of Hardness: 7.5

Sources: Some of the finest Morganite comes from Madagascar. Brazil produces pure pink crystals, as well some interestingly containing both Aquamarine and Morganite in the same crystal.

Other locations include Elba (Italy), Mozambique, Namibia, Zimbabwe and more recently discovered Pakistan.

Also see Beryl

The use of Peridot in jewellery dates back as far as the 2nd Century BC, when is was treasured by the Egyptians. It was also favoured in Medieval times and can be found in many European churches of the period.

Having fallen out of fashion for some time, a resurgence of interest in this golden-green stone occurred in the 1990s, when a deposit of the finest Peridot ever seen was discovered in Pakistan

Mohs Scale of Hardness: 6.5

Sources: Egypt, China, Burma, Brazil, Pakistan, Hawaii, Australia, South Africa and Norway

Colours: Olive – Bottle Green

Birthstone: August

The Plagioclase series is a group of related feldspar minerals that basically have the same formula but vary in their percentage of sodium and calcium.

Each member of the group is defined by its albite and anorthite content. Albite and Anorthite are the end members of the series with the middle members being Oligoclase, Andesine, Labradorite and Bytownite.

The Plagioclase Feldspars are commonly used in ceramics, although there are three varieties that are commonly used as gemstones: Labradorite, Moonstone, and Sunstone.

Mohs Scale of Hardness: 6

Sources: See individual members

Quartz is one of the most common minerals on earth, abounding in an array of varieties of which several are prized as gemstones. Amethyst and Citrine are the most popular and valuable gem varieties, but other forms of Quartz are also used as minor gemstones. Chalcedony is a microcrystalline variety of Quartz, without any visible crystals. Pale, milky coloured varieties are simply referred to as Chalcedony, while others are differentiated by their specific names: Agate, Carnelian, Tiger's Eye, and Chrysoprase, for example.

Quartz is generally a very affordable gemstone, although rare, good quality specimens can fetch high prices. Rock Crystal, Rose Quartz, and Smokey Quartz are sometimes cut as cabochons or hearts, and may also be tumbled for use in bracelets, necklaces, and costume jewellery. Rock Crystal is also commonly cut into large spheres and carvings. More highly prized coloured varieties such as Amethyst and Citrine may be faceted into beautiful gemstones.

Mohs Scale of Hardness: 7

Sources:  Commonly found worldwide (see specific gemstones for more information)

Colours: White, Colourless, Blue, Red, Green, Yellow, Orange, Brown, Pink, Purple, Grey, Black, Multi-coloured

Also see: Amethyst, Citrine, Smokey Quartz, Rutilated Quartz, Ametrine

The most famed and fabled of red gemstones, Ruby is desirable for its bright red colour, durability, lustre, and rarity. Transparent Rubies of large sizes are even rarer than Diamonds. As gem varieties of the same mineral corundum, Ruby and Sapphire are identical in all properties except for colour. While the colour of Ruby ranges from bright red to dark reddish-brown, the most preferred colour is a deep blood red with a slightly bluish hue sometimes known as ‘Pigeon's Blood’. Ruby from Burma is famous for its exceptional colouring, and has traditionally produced the finest Rubies. These rarely exceed several carats, however, and large flawless Burmese Rubies can be worth millions.

In common with Sapphire, inclusions of tiny, slender, parallel Rutile needles cause a polished Ruby to exhibit asterism, and such ‘Star Rubies’ can command very high prices. Opaque or semi-opaque Rubies have relatively little value, however, even if they display asterism.

Mohs Scale of Hardness: 9

Sources: The most valuable Rubies with the finest natural colour come from Mogok, Burma (Myanmar). Other sources include Madagascar, Sri Lanka, India, Cambodia, Vietnam, Tanzania, and Mozambique.

Birthstone: July

This type of Quartz specimen features mineral inclusions that are very common and as a result make attractive gemstones. Rutilated Quartz or ‘Venus Hair’ is colourless with golden yellow Rutile inclusions that are hair-like growths within the gemstone. No two stones look the same, making jewellery set with Venus Hair truly unique. Although inclusions usually detract from the value of a gemstone, with Venus Hair, the more abundant the golden needles, the more desirable the stone.

Mohs Scale of Hardness: 7

Sources: Brazil, Madagascar, South Africa, India, Sri Lanka, Germany and Switzerland

Also see Quartz

Sapphire is a most precious and valuable blue gemstone and very desirable due to its excellent colour, hardness, durability, and lustre. In the gem trade, Sapphire without any colour prefix refers to the blue variety of the mineral Corundum. Nevertheless, the term Sapphire encompasses all other colours of Corundum apart from red, which is Ruby.

The most valuable colour of Sapphire is a cornflower blue, known as Kashmir Sapphire or Cornflower Blue Sapphire. Another extremely valuable type is the rare, orange-pink Padparadscha. An exotic type of Sapphire known as colour-change Sapphire displays a different colour depending on the lighting. In natural light, colour-change Sapphire is blue, but in artificial light, it is violet. (This effect is the same phenomenon well-known in the gemstone Alexandrite). Yellow and pink Sapphire have recently become very popular, and are now often seen in jewellery.

Sapphire often contains minor inclusions of tiny, slender Rutile needles. When present, these inclusions decrease the transparency of a stone and are known as silk. When in dense, parallel groupings, these inclusions can actually enhance by allowing the polished Sapphires to exhibit asterism. Asterism is an effect exhibited by some polished cabochons which causes them to reflect a billowy, star-like formation of concentrated light. Sapphires displaying asterism are known as "Star Sapphires", and may be highly prized.

Sapphire is pleochroic, displaying a lighter and more intense colour when viewed at different angles. Some pleochroic Sapphire is blue when viewed at one angle, and purple at a different angle. Colour zoning, which forms from growth layers that build up during the formation of the stone, may also be present in certain Sapphires, causing different parts of the crystal to be lighter or darker in colour, or even multicoloured.

Mohs Scale of Hardness: 9

Sources: Sri Lanka, Burma (Myanmar), Thailand, Cambodia, Madagascar, Tanzania, Australia, and the U.S. (Montana). The Kashmir region of India/Pakistan was famous for its Kashmir-blue Sapphire, but little material comes from there today.

Birthstone: September

Smokey Quartz is the brown ‘smokey’ variety of Quartz. It ranges in colour from light brown to black. Smokey Quartz has recently increased in popularity in the gemstone trade. The very dark, almost opaque form is sometimes called Morion. Brown or Smokey Quartz from the Cairngorm Mountains of Scotland is appropriately named Cairngorm.

Mohs Scale of Hardness: 7

Sources: Brazil, Argentina, Switzerland, Italy, Malawi, Namibia, Australia, United States and Scotland

Spectrolite is a variety of Labradorite which displays intense reds, oranges and yellows and even blues and greens. The colour play in the stone is glistening like the feathers of a peacock or the Northern Lights.

According to beliefs, spectrolite is said to bring strength, originality, aid sleep and good luck. It is also said to provide immediate relief from anxiety, hopelessness and depression. On the positive side it is believed to provide enthusiasm, self-confidence and inspiration. People also call it "The good luck stone."

Mohs Scale of Hardness: 6

Sources: Labrador (Canada), Madagascar, India, Finland and Russia.

Also see Labradorite

For many years, Spinel was thought to be a variety of Ruby. The magnificent red stone that adorns the Imperial State Crown in the Crown Jewels, known as the Black Prince’s Ruby, is in fact a Spinel.

Spinel comes in a wide array of colours and has spectacular brilliance and hardness. Although a fine Spinel is now rarer to find than a Ruby, it is paradoxically more affordable. Grab one of these unsung heroes whilst you can!

Colours: red, grey, mauve, pink, blue and many more!

Mohs Scale of Hardness: 8

Sources: Burma, Sri Lanka, Madagascar, Afghanistan, Pakistan, Brazil, Australia, Sweden, Italy, Turkey, the former USSR, USA

Tanzanite is to be found in only one place in the world, the country after which it was named: Tanzania. Since its discovery in the 1960s, Tanzanite’s popularity has soared, helped, no doubt by the jewellers, Tiffany and Co who championed it. The colours of Tanzanite range from ultra-marine blue to a light violet blue.

Tanzanite is a relatively soft stone and is therefore not recommended for use in rings that will be worn everyday, as constant wear will cause the facets to dull quickly. While facets can be re-polished, the need for this means rings set with Tanzanite are high maintenance! Nevertheless, this stone will dazzle in a cocktail ring, pendant or earrings.

Mohs Scale of Hardness: 6.5

Sources: Tanzania

Birthstone: December

Deep golden yellow Topaz and pink Topaz are the most valuable colours of this stone. The blue Topaz with which we are familiar today actually started out as white Topaz. Once irradiated and heat treated, the stone changes in colour to a range of blues that often makes it hard to distinguish from Aquamarine.

Like the gemstone Spinel, Topaz has an inadvertent regal history. In the 17th Century the 1,4640 carat ‘Diamond’ set in the Portuguese Braganza crown is now believed to have been a Topaz.

The light blue to blue-green colour of Topaz may fade upon prolonged exposure to light, so it is especially important to purchase this gem from a reliable dealer.

Mohs Scale of Hardness: 8

Sources: Brazil, the USA, Sri Lanka, Burma, Australia, Tasmania, Pakistan, the former USSR, Mexico, Japan and Africa

Birthstone: November

The myriad colours of Tourmaline have resulted in its being known as the 'gemstone of the rainbow'. Its colours range from blue-green and pink-yellow, and almost everything in-between!

The Paraiba Tourmaline is the most prized variety of the stone. Its exquisite blue-green colour is not found in any other natural gemstone, and has made it highly sought-after amongst collectors and jewellers all over the world.

Not only do Tourmalines come in many colours, individual stones can also be bi-coloured or even tri-coloured. Perhaps the most well known of these is the Watermelon Tourmaline, so called because of its pink centre and green rim.

Mohs Scale of Hardness: 7.5

Sources: Madagascar, Brazil, USA, Burma, and East Africa

Also see Watermelon Tourmaline

Birthstone: October

These are a specific type of Tourmaline that usually occur with a pink centre and a green rim, or vice-versa. They are called Watermelon Tourmaline because their colouring is similar to the pink flesh and green rind of a watermelon.

Mohs Scale of Hardness: 7.5

Sources: South Africa, East Africa and Brazil

Birthstone: October
 
Also see: Tourmaline

Natural Zircon is often and wrongfully confused with the artificially produced Cubic Zirconia. Natural Zircon is a beautiful stone in its own right, and the dazzling colourless variety looks more like Diamond than any other natural stone.

Zircon also comes in hues of green, red, brown, orange and blue.

Mohs Scale of Hardness: 7.5

Sources: Sri Lanka, Burma, Thailand, Cambodia, Vietnam, Kampuchea, Australia, Brazil and Nigeria

Zultanite is an exciting new gem stone, which changes colour when viewed in different kinds of light. Its natural unenhanced colour ranges from chocolate to mocha to golden green. Zultanite is an exclusive and rare gem, as there is only one mine suitable for the ethical mining of the stone and the amount of fine quality yield is limited. Zultanite is not only rare but also very new, the first stone having been sold in 2006.

If you’re seeking an unusual or unique stone for your collection or wish to  make a very special piece of jewellery, then Zultanite is the stone for you. As one of the few places in the UK selling fine quality Zultanite, Holts will be delighted to help you with your acquisition.


Family: Diaspore

Chemical formula: ALO (OH)

Crystal Structure: Orthorhombic

Hardness: 6.5-7

Refractive Index: 1.70-1.75