A perfect gemstone is a flawless, transparent crystal that sparkles brilliantly as it reflects light internally. Clarity is the quality most prized in gemstones.Gemstones are valued according to four different criteria: clarity, colour, cut and carat (weight or size). They are not all rare, and most have a hardness of less than 7 on Moh’s scale. This term is used to describe minerals that lack transparency, but have attractive colours, textures and patterns such as jade, malachite, chalcedony and lapis lazuli. Some of these properties are qualitative, so can be described rather than measured and some are quantitative and can be measured using appropriate optical instruments.Īnother term sometimes used is ‘ornamental gemstone’. In addition, in some gems there is dispersion of light or 'fire'. A gem can be coloured or have changing colour patterns, differing levels of transparency, lustre and brilliance. The 'beauty' of a gemstone is evaluated by examining how light is transmitted or refracted through the gem or reflected from the gem's surface. Gemmologists use grade as a general measure of gemstone quality, using the 4Cs (clarity, colour, cut, and carat) to determine the potential price. Precious gemstones are not always rarer or more valuable, than semi-precious gemstones. Many people in the gem and jewellery industry do not like the terms precious and semi-precious, because they do not take into account the grade of the gemstone. Some Australian examples of these gems can be seen at Google Arts and Culture - Gems from the Safe. Gemstones that were referred to as semi-precious are used in jewellery and ornaments. Diamonds, rubies, sapphires, and emeralds were originally considered the ‘precious stones’, but sometimes this category included opal, jade, or pearls. However, these divisions are not scientific and have never been truly meaningful. In the mid-1800s, gemstones were first classified as either ’precious’ or ‘semi-precious’. Precious, semi-precious or ornamental stones A two carat diamond is larger than a two carat cubic zirconia and very much more expensive. This means that cubic zirconium is heavier than diamond. For example, diamond has a specific gravity of 3.52 and a cubic zirconia, which looks very similar, has a specific gravity of 5.80. Most gemstones are harder than quartz (Mohs scale greater than 7) and cannot be scratched by the blade of a knife. Using properties of minerals such as habit, shape, lustre, light refraction and specific gravity we can tell the difference between similar looking gemstones. The test of a good gemstone is its resistance to wear and tear. Information on some of Australia's gemstones can be found at Google Arts and Culture - Gems from the Safe. Australia is the world’s biggest producer of diamonds and opals and major supplier of sapphire, ruby, emerald, garnet, topaz and jade has also been mined in Australia. Australia, with its long geological history, has some of the world’s oldest rocks and minerals and a wide variety of gemstones.
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