A-Z of Gemstones: T

Posted on the 1st November 2016

A-Z of Gemstones: T: Tanzanite, Topaz, Tourmaline, Turquoise 

ΤΑΝΖΑΝΙΤΕ

The origin of the name Tanzanite comes from the country in which it was first discovered, by Tiffany & Co in 1967, and is the only known source of the stone: Tanzania.

Tanzanite belongs to the zoisite species and it comes in striking purplish blue to blue hues.

Tanzanites are usually transparent to translucent and they are cut into faceted gems, cabochons and beads.

Normally tanzanite looks very clean to the naked-eye.

Tanzanite’s lustre is vitreous; it rates 6-7 on the Mohs scale and has cleavage planes which make it suitable only for jewellery that has a secure setting to protect the stone from knocks.

Normally, tanzanites are very clean stones free from inclusions and they have a characteristic strong pleochroism displaying blue, purple and pale greenish yellow colours.

The main and only source of Tanzanite as mentioned is Tanzania in eastern Africa.

Most tanzanites’ colour is the result of heat treating zoisite where yellowish and brownish stones become an amazing blue and purplish blue.

Synthetic tanzanite is not yet available but there are some materials that are used to imitate its’ qualities such as synthetic forsterite, glass and plastic.

Tanzanite is the December birthstone.

TOPAZ

Topaz is mostly known as a blue stone but it actually comes in a wide variety of colours.

Topaz can be blue, yellow, orange, brown, pink, red, purple, green or colourless, cause by color-centres or chromium traces.

The most valuable variety of topaz is known as “Imperial Topaz”: a vivid orangey red to reddish orange stone.

Depending on the transparency of the stone, which can be anything between transparent to semi-translucent, topaz is mostly cut into faceted stones, cabochons and beads.

Usually, topaz looks very clean to the naked eye.

Topaz has a vitreous to sub-adamantine lustre; it rates 8 on the Mohs scale but because it has a cleavage plane it’s mostly suitable for jewellery that has secure settings to protect the stone from knocks.

The main sources of topaz are: Brazil, Pakistan, Sri Lanka, Australia, Madagascar, Mexico, Myanmar, Namibia, Nigeria and the U.S.A.

Topaz is very commonly treated; it undergoes heating and sometimes irradiation in order to create a more desirable colour.

Materials that are most commonly used to imitate topaz are glass and plastic.

Topaz is the November birthstone.

TOURMALINE

Tourmaline’s name derives from the Sinhalese word “toromalli” which means “mixed gems”.

Tourmalines are a wide group of closely related mineral species and it is probably the stone with the widest variety of colours and vividness; they come in pink, green, blue, yellow, brown and colourless.

The most valuable variety of tourmaline is the Paraiba which is praised for its unique neon blue to greenish blue striking colour.

Other desirable varieties are the vivid purple rubellites and the bi-coloured watermelon tourmalines that are pink, green and some have an additional colourless section inbetween the two colours.

Tourmalines might display a phenomenon called chatoyancy that refers to the cat’s-eye appearance of some cabochon-cut stones.

Depending on the transparency of the stone which can be anything between transparent to opaque, tourmalines are cut into faceted stones, cabochons, beads and occasionally rough crystals are left intact in their hosting rocks and sold as gorgeous rough specimens.

Tourmaline has a vitreous to sub-adamantine lustre; it rates 7-7.5 on the Mohs scale which makes it suitable for jewellery that has secure setting to protect the stone from wear.

The main sources tourmalines are: Brazil, Nigeria, Zambia, Mozambique, Madagascar, Namibia, Tanzania, Afghanistan, Pakistan, Russia and the U.S.A.

Tourmalines can be heat treated or irradiated in order to enhance their colour.

Synthetic tourmalines are not yet available on the market.

Materials that are most commonly used to imitate tourmalines are glass and plastic.

TURQUOISE

Turquoise is a semi-translucent to opaque stone praised for its vivid blue colour.

Turquoise can vary from blue, greenish blue to yellowish green; iron elements usually produce a purer blue whilst traces of copper produce a more greenish blue.

Some types of turquoise have a black or dark brown matrix usually recessed from the surface.

Turquoise is usually cut into cabochon and beads or carved into decorative ornaments.

Turquoise has a waxy to dull lustre; it rates 5-6 on the Mohs scale which makes it suitable only for jewellery that has secure settings to protect the stone from scratching.

Historically turquoise was mined in what is now Mexico and Central America as well as Persia, known today as Iran. Nowadays the main sources of turquoise are China and Arizona U.S.A.

Turquoise is a porous stone making it more vulnerable compared to other gemstones so it is commonly treated with oil, wax or polymer plastic that is impregnated in the stone and improves its stability. Another less common treatment is dying which enhances the stone’s colour giving it and unnatural blue.

Turquoise has been synthesized since the 1980’s and but it has a significantly lower value than natural turquoise so it must always be disclosed as synthetic.

Materials that are most commonly used to imitate turquoise are: glass, plastic, dyed howlite and reconstructed“turquoise”; a synthetic material made of dyed plastic or epoxy resin.