A-Z of Gemstones: S

Posted on the 2nd November 2016

A-Z of Gemstones: S: Sapphire, Spinel, Sunstone

SAPPHIRE

The name “sapphire” derives from the word “sappheiros” in ancient Greek and is a variety of the corundum species.

“Sapphire” always refers to stones that vary from violetish blue to slightly greenish blue and its primarily the presence of iron and titanium trace elements that produce these beautiful blues. The most valuable colour of sapphire is what is known as the “cornflower blue” or “Kashmir” which is a strong pure blue to violetish blue and sometimes has a velvety look.

Any other colour, besides red which would make it a ruby, is considered a fancy sapphire such as yellow sapphire, green sapphire and pink sapphire.

Within the fancy sapphire range the rarest and most prized colour is the padparadscha which means “lotus flower” in Sinhalese. The stone is found in Sri Lanka and has a vivid orangey pink to pinkish orange stiking colour which is caused by iron and chromium trace elements or by colour centres. 

One interesting thing that you might find in sapphires is an inclusion also knows as “silk” consisting of fine rutile needles that give a velvety-milky look to the stone.

Some sapphires can display what is called in gemmological terms, a “phenomenon”. Two of the most important types of phenomenal sapphire are colour-change sapphires and star sapphires. The former change colour when seen under a daylight equivalent light source and under incandescent lighting and the latter are cabochon-cut stones that display a whitish star created by fine intersecting needle inclusions.

Depending on the transparency of the stone which can be anything between transparent and opaque sapphires are cut into faceted stones, cabochons, beads and carvings.

Sapphire’s lustre is vitreous to sub-adamantine; it rates 9 on the Mohs scale and therefore it is ideal for jewellery that is worn frequently.

Mines that have yielded the finest sapphires in colour and clarity are located in Kashmir, Myanmar and Sri Lanka. Nowadays the main sources of sapphires are: Thailand, Cambodia, Australia, Madagascar and Montana in the U.S.A.

Sapphires are often treated to enhance their colour or improve their clarity; some of the most common treatments are: heat (with or without the use of chemicals), lattice-diffusion and beryllium-diffusion.

Synthetic sapphires have the same chemical properties to the natural sapphires but their value is significantly lower so they should always be disclosed as synthetic.

Some of the materials that are mostly used to imitate sapphires are: glass, garnet-and-glass doublet, synthetic spinel triplet and sapphire and synthetic sapphire doublet.

The largest faceted sapphire in the world is the “Blue Giant of the Orient” which has an intense medium blue colour and is cut into a cushion shape. It weighs a huge 486.52cts.

Sapphire is the September birthstone.

 

SPINEL

Spinel used to be offered as a ruby alternative but nowadays it is praised for its own special characteristics.

Spinel can be red, pink, orange, purple, violet or blue. Red and pink are caused by traces of chromium, orange and purple by chromium and iron and dark blues by traces of iron and cobalt.

Some spinels display phenomena such as colour-change and asterism.

Depending on the transparency of the stone which can be transparent to translucent, spinels are cut into faceted gems, cabochons and beads.

Usually, spinels look clean to the naked-eye.

Spinel lustre is vitreous to sub-adamantine; it rates 8 on the Mohs scale which makes it suitable for jewellery that is worn frequently.

The main sources of spinels are: Sri Lanka, Afghanistan, Myanmar, Tanzania and Madagascar.

Spinel is not usually treated or enhanced.

Synthetic spinels are readily available on the market and they are also used to imitate other stones such as sapphires. Synthetic spinels have the same chemical properties to natural spinels but their value is significantly lower so they should always be disclosed as synthetic.

SUNSTONE

Sunstone is a phenomenal variety of the orthoclase, oligoclase or labdradorite species that belong to the wide group of feldspars. 

Sunstones display a phenomenon called aventurescence which refers to the glittery effect caused by tiny metallic inclusions inside the stones.

Depending on their variety, sunstones can have a yellow, orange, brown, light green or almost colourless body colour and the included flakes are usually hematite or copper.

Sunstone’s lustre is vitreous; it rates 6-7.2 on the Mohs scale which makes it suitable for jewellery that has secure settings to protect the stone from scratching.

The main sources of sunstones are: Norway, Sweden and Oregon U.S.A

There are no known treatments for sunstones and it has not yet been synthesized.

The material that is most commonly used to imitate sunstone is called goldstone glass.