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Gemstone Learning Guide

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Jewellers describe the colour of a gemstone in terms of three characteristics: hue, saturation, and tone. A gemstone's basic colour is its hue, and those with purer hues (for emeralds, green; for sapphires, blue; and for rubies, red) are generally considered more valuable. Often, however, a hint of another colour can be detected. Sapphires will range from purplish blue to greenish blue; emeralds from yellowish green to bluish green; and rubies from orangey red to purplish red. Saturation is a measure of the intensity or purity of a gem's hue and is determined by the degree to which grey or brown hues mute its defining colour. Value tends to increase with saturation, so a fully saturated purplish blue sapphire may well be more expensive than a muted pure blue one. The tone of a gemstone, a measurement of its lightness or darkness, is usually given as light, medium-light, medium, medium-dark, or dark.

A flawless gemstone is rare and usually extravagantly priced. As with diamonds, most gemstones have inclusions, or tiny mineral flaws, that can be seen under magnification or by the careful eye. Sapphires tend to be moderately included, while emeralds and rubies are usually heavily to moderately included. Clarity grades range from VVS (very, very slightly included) to I3, in which inclusions are prominent and severely affect the gemstone's beauty.

A gemstone's cut refers to its proportions and symmetry and is essential in making a gemstone look appealing. The stone should be symmetrical in all dimensions so that it will appear balanced and ensures that its facets will reflect light evenly, providing good brilliance. Colour should also be taken into account when cutting for optical efficiency. If a stone's colour is highly saturated, a shallow cut will allow it to pass more light, while a deeper cut may increase the vividness of a less saturated gem. There is no generally accepted grading system for gemstone cut.

Though a gemstone's weight is usually given in carats, this may not give an accurate idea of its size, because different types of stone have different densities. Therefore, a 1-carat sapphire or ruby will be smaller than a 1-carat emerald, though they have the same weight, because sapphires and rubies (both a form of the mineral corundum) are more dense than emeralds.
Gemstones can also be measured in dimensions (diameter, length, and width). It is important to know the dimensions to ensure that the gemstone weight will be visible and well proportioned in the setting.

Gemstones are often treated to enhance colour and fill cracks. Please note, that if a gemstone has been treated, this information will be provided in the specifications on the stone's product detail page. There are various accepted techniques, depending on the type of stone. For example, oils and resins are frequently used to fill surface-reaching fissures in emeralds and rubies. This process is not permanent, however; if a treated ruby or emerald is subjected to high heat from a jeweller's torch, an ultrasonic or steam cleaner, or even hot dishwater, the filler may leak out of the stone. Sapphires and rubies are often heated in a kiln or furnace to enhance their colour. Such heat treatment, which is considered permanent, has been used for centuries. Lightly coloured sapphires may also undergo diffusion treatment. This is done by heating a stone in the presence of colouring elements such as titanium or iron, which causes a thin layer of colour to be diffused into the stone's surface. Because this colour layer is so thin, a diffusion-treated sapphire should not be repolished.

To clean gemstone jewellery, first wipe it with a soft cloth to remove any dirt. Emeralds require only the use of a soft cloth, as cleaners may damage these soft gems. Sapphires and rubies can be cleaned with either an ultrasonic cleaner or a solution of one part ammonia to six parts water, with a soft brush. Store your gemstone jewellery in a soft cloth pouch or jewellery case so it does not touch other pieces in your collection. As with all jewellery, normal wear and tear can loosen prongs and settings, so it is a good idea to have it checked by a professional jeweller who can make repairs as needed.