Gemstone Overview

All precious and semi-precious stones have something special and beautiful about them. Value is given to gemstones according to availability, aesthetics, colour, transparency, lustre, and hardness. Semi-precious stones often refer to softer stones of less value, as opposed to precious stones which are harder and have more value. This terminology however, is not entirely correct as many “semi-precious” stones are more valuable per carat than “precious” stones. It would be better to refer to them all as “gemstones”.

Availability of gemstones is not always as straight forward as what is being mined at the time. In some cases the “availability” of gemstones is controlled or ”managed”. For example, the value of a red garnet (pyrope garnet) is less than an orange garnet (spessartite). This is because the orange garnet is far less common, plus it has the added value of an intense orange colour that is highly sought after.

The Gems among gemstones:

The Beryl group
In the Beryl group there are emeralds, aquamarine, yellow, yellow/green, pink, and colourless Beryl. They all have the same chemical composition with different trace elements causing the different colours. Beryls can be faceted or cut en cabochon.

The Emerald is the most rare of the Beryl group, and therefore more expensive. One very seldom finds large, clear emeralds. There are some famous large emeralds, but these would be part of the crown jewels or national collections you find in the London Museums. They are a very specific green colour, and the term “emerald green” is not only used for gemstones. The name comes from the Greek word – Smaragdos, meaning green stone. Emeralds are very rarely free of inclusions, bubbles, healing cracks or foreign crystals. These inclusions are known as the “jardin” or” gardens”. The inclusions cause internal stresses, and combined with the brittleness of emeralds, means emeralds have to be worn with extreme care. They are sensitive to pressure and temperature change (so washing dishes is out of the question!)

The Corundum group
Rubies and sapphires are part of the corundum group. On the Mohs scale of hardness they are 9. The jump in hardness between corundum and diamond (Mohs 10) is much more than it seems from the scale. A diamond is 140 times as hard as corundum, but corundum is very hard wearing… seven times as hard as topaz. Corundum can be cut either en cabochon or faceted.

The red corundum is called ruby, from the Greek name for red – rubeus. There is no demarcation between ruby and sapphire – they are the same chemical composition but the colour pigment influencing ruby is chrome. The most sought-after colour is “pigeon blood” red – pure red with a slight blue hue.

Any stone referred to as a sapphire is blue, all the other colours are qualified by description i.e. yellow sapphire, green sapphire, pink sapphire, violet sapphire, and red sapphires. The colouring pigment in blue sapphire is iron and titanium; in violet stones, vanadium. A small iron content results in yellow and green tones, whereas chrome produces pink. The most highly sought after colour is “cornflower blue” which used to be called “Ceylon Sapphire”.

Tanzanite is the blue version of Zoisite, which is also found in green and red. Tanzanite was first discovered in 1967 in Tanzania, hence the name. It was first used by New York jewellers Tiffany & Co, but since then a marketing genius has put it on the world map. The marketing has been so good that most people have heard of it, and would like to own one. It has an incredible intense colour, but is very soft. Other semi -precious stones which have incredible colour but are soft, do not hold a good value, but tanzanite, despite its softness is almost as expensive as a good sapphire, which is hard and can be an incredible cornflour blue. This can be due only to the marketing drive. Most tanzanite comes out of the earth brown, and are heat treated to be blue, at temperatures of 400-500C. If the blue is not intense enough, it is “colour enhanced”. There is no problem in heat enhancing stones, as long as the client who is purchasing the stone knows what they are buying. They need to know if a stone is naturally coloured or not. I would discourage anyone to wear a Tanzanite in a ring, unless it is purely a dress ring. Wear it preferably as a pendant or earrings. If you asked any setter in the trade what they thought of tanzanite, they would probably like to kill the genius that put it on the map!

Precious topaz comes in yellow, red-brown, pinky red, pale green, colourless and light blue. The light blue stone is called “Sky Blue”, a more intense blue (which is heat-treated) is called “Swiss Blue”, and then finally the most intense teal blue stone (also heat-treated) is commonly known as “London Blue”. Most people would not be able to tell the difference between aqua and Sky Blue topaz, as they are very similar in colour, and possibly a little harder, yet is still sensitive to pressure with strong cleavage – so is a very good substitute for aquamarine. Blue is a always good seller – traditionally in the jewellery trade, blue sells the best – it is well-priced and very attractive. Keep that in mind for resale! This gemstone is found naturally in the the softer colours, such as pale green (not too different from peridot) and a very pretty salmon pink (the most valuable). Many people refer to citrine as golden topaz, or smoky quartz as smoky topaz, which is not correct, as these are also both part of the quartz family. Stones with very intense colour have been heat-treated – the very common yellow stones turn pink or blue when heated. Most topaz is mined in Brazil, and the Far East, with smaller deposits in many other parts of the world.

Tourmaline is unique because of its amazing variety of colours. The crystal grows gradually changing colour. Another incredible fact about tourmaline is that depending on where it is from, the colour range changes within the crystal, Brazil produces crystals that grow from red to green on the outside rather like a watermelon, and Southern Africa produces crystals that grow from green to red on the outside. It is very unusual to find an entire crystal of tourmaline in one colour. This is why it is very unusual and difficult to find a perfectly matching pair of tourmalines of any significant size If there is a large piece of crystal, it would more often than not be cut into one large stone because it can be sold for much more per carat, as opposed to cutting it into a pair. Another reason that it is so difficult to match, even if you have one and are trying to match it.