Gemstones are beautiful, unique and truly fascinating.
Although there are many factors which should be considered when purchasing gemstones, colour is thought to be the most important. Saturation, depth of tone and hue are the key factors that determine the gemstone’s value and should be considered when making a precious jewellery purchase.
The tone, colour (known as hue) and saturation are thought to be the most important factors to consider when purchasing gemstones. Some hues are more valued than others, for example, sapphires in the velvet-blue to violet-blue range are thought to be ideal. Similarly, sapphires very light or very dark in colour are considered less valuable.
The GIA (Gemological Institute of America) developed a colour grading system for coloured gemstones. This includes 31 different hues ranging from red to brown, with gradations such as slightly yellowish green and very strong greenish blue.
Tone refers to how light or dark a gemstone is, and is another incredibly important factor as it affects the gemstones hue. For example, a sapphire that has a medium to medium-dark tone is more valuable than one which is extremely light or dark in tone.
The definition of clarity is similar amongst both diamonds and coloured gemstones. Clarity refers to the amount of flaws on the given gemstone. Internal flaws are inclusions and external flaws are generally referred to as blemishes.
It is important to understand that most gemstones have some amount of inclusions, finding a flawless coloured gemstone is actually rarer than finding a flawless diamond. The process within the earth which creates them determines the number of flaws a stone will have. For example, due to the violent circumstances under which emeralds are formed, these are almost always included. Darker gemstones tend to be able to carry more inclusions without lessening their value because the depth of colour masks most flaws. Likewise, lighter gemstones show internal inclusions much more clearly so it is best to consider this when purchasing.
The GIA grade clarity of coloured gemstones use three categories; Type I, Type II and Type III.
Type I – Inclusions are not noticeable to the untrained eye, this type of gemstone has formed under geological conditions.
Type II – Inclusions are typically slightly noticeable, this type of gemstone has formed under geological conditions that are more severe.
Type III – Inclusions are typically noticeable, this type of gemstone has formed under geological conditions that are very violent.
Most gemstones today have been treated to enhance their colour and clarity to improve their appearance. In fact, gemstones that naturally have a beautiful colour and clarity command extravagant prices. There are a number of ways in which coloured gemstones can be treated which range from heating, bleaching and irradiation.
The application of heat enhances the colour and clarity of coloured gemstones and tends to be part of a standard polishing process. The changes made as a result of this are permanent. This treatment has been common around the globe for centuries and is accepted by the jewellery industry.
Founded by gemstone merchants, this process began centuries ago when they discovered that immersing emeralds in oil or waxes made them appear clearer and more desirable.
This practice is still common today, where coloured gemstones are filled with oil, resin, wax or other materials to improve their appearance to the untrained eye.
In order to improve the sturdiness and appearance of coloured gemstones, the use of wax, resin or oil is applied to the exterior to protect the natural substance.
Bleaching is the application of chemicals or other elements in order to lighten or enhance colour consistency on particular gemstones.
This is where colour agents are added so they permeate a gemstone to alter or enhance the colour.
A permanent treatment where radiation is used to alter and enhance a gemstone’s colour to make it appear more desirable. This is often followed by a heating process.
Contrary to popular belief not all sapphires are blue - they can come in every colour under the rainbow (apart from red). Blue sapphires are generally referred to simply as sapphires and sapphires of other colours are named yellow sapphire, pink sapphire etc.
Sapphires get their signature colour from traces of iron, chromium, titanium and other elements. Colour is considered the most important aspect when estimating the value of sapphires. Hues closer to a pure blue are deemed more desirable, but saturation is more important with top sapphires reaching vivid saturation. Dark sapphires are deemed less desirable and this is represented in their value.
Similar to other coloured gemstones, colour is considered the most important factor when pricing emeralds. The ideal emerald would be saturated and bright, with a bluish-green to green hue. Emeralds which are highly transparent with even colour distribution are deemed most prized.
Unlike diamonds, the clarity of emeralds are graded by eye as numerous inclusions and surface breaking fissures are to be expected with this gemstone. Therefore if an emerald has no visible inclusions to the eye it is considered flawless - this is incredibly rare. Due to this, most emeralds are treated to enhance clarity.
Rubies and sapphires are in fact the same mineral - just different colours.
Rubies receive their incredible red hue as a result of high levels of chromium. As with most coloured gemstones, colour is by far the most important thing to consider when evaluating a ruby. The lines between colour boundaries of red and pink are often blurred for rubies. The ideal colour is considered a vivid, medium-dark red to slightly purplish red with a medium to medium-dark tone.
Similar to sapphires and emeralds, a ruby’s clarity is not graded under 10x magnification as inclusions are to be expected with coloured gemstones. Instead, they are graded by eye, with a high clarity ruby considered ‘eye clean’.
Unlike diamonds, there isn’t an ideal cut for coloured gemstones. The cutter usually assesses each stone individually and works around flaws, as there is no fixed rule for cutting coloured gemstones. The best-considered cuts are those which enhance its natural beauty by reflecting light in an even manner without darkness or windowing the stone. This results in coloured gemstones which come in a variety of shapes and sizes.