Australian gems

Introduction

Australia, with its long geological history, has some of the world’s oldest rocks and minerals and a wide variety of gemstones. 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. Information on some of Australia's gemstones can be found at Google Arts and Culture - Gems from the Safe.

The test of a good gemstone is its resistance to wear and tear. Using properties of minerals such as habit, shape, lustre, light refraction and specific gravity we can tell the difference between similar looking gemstones. Most gemstones are harder than quartz (Mohs scale greater than 7) and cannot be scratched by the blade of a knife. 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. This means that cubic zirconium is heavier than diamond. A two carat diamond is larger than a two carat cubic zirconia and very much more expensive.

Precious, semi-precious or ornamental stones

In the mid-1800s, gemstones were first classified as either ’precious’ or ‘semi-precious’. However, these divisions are not scientific and have never been truly meaningful. Diamonds, rubies, sapphires, and emeralds were originally considered the ‘precious stones’, but sometimes this category included opal, jade, or pearls. Gemstones that were referred to as semi-precious are used in jewellery and ornaments. These include:

• agate
• amber
• amethyst
• aquamarine
• aventurine
• chalcedony
• chrysocolla
• chrysoprase
• citrine
• garnet
• hematite
• jade
• jasper
• jet
• kunzite
• lapis lazuli
• malachite
• moonstone
• obsidian
• onyx
• peridot
• rhodonite
• sodalite
• sunstone
• tiger's eye
• tanzanite
• topaz
• turquoise
• tourmaline

Some Australian examples of these gems can be seen at Google Arts and Culture - Gems from the Safe.

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. Precious gemstones are not always rarer or more valuable, than semi-precious gemstones. Gemmologists use grade as a general measure of gemstone quality, using the 4Cs (clarity, colour, cut, and carat) to determine the potential price.

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. A gem can be coloured or have changing colour patterns, differing levels of transparency, lustre and brilliance. In addition, in some gems there is dispersion of light or 'fire'. 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.

Another term sometimes used is ‘ornamental gemstone’. 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. They are not all rare, and most have a hardness of less than 7 on Moh’s scale.

The 4 Cs for gemstones

Gemstones are valued according to four different criteria: clarity, colour, cut and carat (weight or size).

• Clarity is the quality most prized in gemstones. A perfect gemstone is a flawless, transparent crystal that sparkles brilliantly as it reflects light internally. Sometimes crystals contain inclusions which are impurities that distort the appearance of the gemstone. Some gemstones, such as star sapphires, pink diamonds and rutilated quartz are valued even more because of these inclusions. Inclusions can be used to identify if a gemstone is naturally formed or synthetically made. Clarity ranges from internally flawless to imperfect.
• Bright and intense colour will increase the value of a gemstone. Colourless beryl is only moderately valued, but emerald (green beryl) is one of the world’s most valued stones. Jade, turquoise and lapis lazuli have rich green and blue colours making them hugely sought after. Many gemstones acquire their colours due to their chemical composition and trace elements or impurities contained in the stones. Peridot (gemstone olivine) is commonly green due to its chemical composition, but can vary from pale lemon to dark olive green. Colourless diamonds are usually the most highly valued, however, diamonds tinted blue or pink from impurities are sometimes more valuable because they are so rare; many of the diamonds from the Argyle diamond mine in Western Australia are pink or champagne coloured, increasing their value.
• Most gemstones which are used for jewellery have been cut or faceted. If cut incorrectly gemstones will have less sparkle and consequently be poorer quality.
• The term carat refers to the weight of a gemstone. One carat = 200 milligrams (1/5 of a gram).

Formation

Unusual geological conditions are required to create gemstones, which is why they are so rare. Gemstones are often found in igneous rocks. Pegmatite, an intrusive igneous rock, may concentrate rare minerals to form gemstones such as beryl, ruby, sapphire, tourmaline and topaz. Intense metamorphism may create garnet, emerald, jade and lapis lazuli.

Glossary