Characteristics of precious stones
The value of precious stones are determined by a number of characteristics. There are some characteristics that are commonly attributed to all stones, but there are some that are only applied to certain precious stones. The overall value of a precious stone is usually determined by its beauty, durability and rarity. Other characteristics that play a role in the value of a gemstone are certain special effects or phenomenon that sometimes occur.
The beauty of a precious stone is determined by its colour, its sparkle, its size and its shape. Size and shape is usually determined by the gem cutter and are also affected by what is available to the market. Perhaps on the more important aspects, at least for the shopper, is the colour. The colour of a precious stone is determined by its chemical composition and crystal structure. For example, both rubies and sapphires are corundums, but rubies contain iron and chromium impurities that cause the red colour. Sapphires, on the other hand may contain titanium or other elements that create its colour.
The sparkle of a precious stone is determined by its refractive index and its lustre (or reflection). The refractive index refers to the amount by which light is refracted by the stone. A simple explanation is that light passes at different speeds through different materials. When the light enters another medium, the path of the light changes – it is refracted. How much the path of the light is changed depends on the medium that it is passing from or into – that is, the stone.
The refractive index of a stone will affect how the light is reflected inside the stone as well as the dispersion affect of the light when it leaves the gem. Dispersion, also known as the fire of the stone, refers to how much the light is broken up when it is refracted. This is the rainbow-effect that sometimes occurs when light passes through a gem. Diamonds are well known for their characteristic fire. The cut of the gem also effects the amount of fire and also the lustre of a stone.
Almost all gems are cut before they are used in jewellery or sold to the public. The main reason is because it greatly enhances the visual appearance of the precious stone. There are two main kinds of cuts: cabochon and facet. Cabochon gems are cut into a dome shape while facet gems are cut to certain angle. In a facet cut, the angles are very important. The effect that most gem cutters aim for is an angle that allows all light that enters to leave through the top.
The reflection of a precious stone is more commonly known as the lustre. The lustre of a stone is a combined function of the smoothness of the surface, the refractive index and the cut of the stone. There are a variety of terms used to describe the lustre of a stone including adamantine, pearly, metallic, silky, vitreous, resinous and waxy, to name a few.
Certain special effects or phenomenon can also improve the worth and beauty of a precious stone. Usually impurities in the structure of a stone detracts from its value, but there are certain instances where it adds to the value.
Chatoyancy (cats eye) is an effect that occurs when impurities in the are present in fibres along a crystal axis. If cut correctly, these lines can show up as narrow, reflective bands of light. The result is an effect that sometimes resembles a cats eye. Related, and rarer, is asterism. Impurities in fibres align to several crystal axes. If the stone is cut into a cabochon, the result is a luminous, star-shaped design on the gem. Star sapphires are much sought after asterised stones.
Aventurescence is also caused by impurities in the structure of the precious stone. In this case it is caused by small particles of another material embedded in the crystal. While considered a flaw, in some cases, it causes a glittering effect that is quite beautiful and sought after.
After beauty, durability is another important factor in the value of precious stones. Durable gems are much more desired than for jewellery and industry. A stone's durability is determined by its cleavage and hardness. Cleavage refers to a stone's tendency to split along certain planes. This is useful when identifying a gem or trying to cut a larger stone into smaller ones without damaging the natural structure. Hardness is a relative term that is defined by Moh's scale.
The hardness of precious stones are defined on scale of one to ten, with one referring to talc and ten referring to diamond. According to this scale a stone higher on the scale is able to scratch or mark a stone below it. Note that a hard stone is not necessarily a more durable one. Diamond may be one of the hardest natural materials known to man, but it can easily be shattered – especially if hit on the right spot. The general rule is that the harder a gemstone, the more brittle it is, but exceptions do exist.
Finally, rarity is becoming a bit of an outdated system for value. Where precious stones were once limited to what could be found, new mining techniques along with the ability to synthesise most precious stones has made it easier to make these gems available. Still, fashion, desire, size and special effects, like the ones mentioned above, do play a role in the value of many precious stones. It goes without saying that gems that are currently in fashion are often more valuable. Large gems, also tend to be worth more, but only to a certain point. After all, very large precious stones are of little to no use in most jewellery.
Yet, despites the advances in technology, there are still some stones that cannot be synthesised and that remain hard to find. These stones can sometimes be worth more than diamonds if they have the right right shape, size and clarity.
Precious stones have been used by people for ages to decorate and impress. Sometimes their values are hard to understand. Diamonds are rather boring in their colourlessness yet are worth a fortune, while amethysts that have a range of beautiful purple shades, are usually ignored. The value of gems is based on the characteristics above, that is for certain, but human whim also plays a large role.
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