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Interpretation of colour is subjective
Interpretation of colour is subjective

Colour Investigation: Ruby

Desire for ruby today is as great as ever. With fluctuating quality and supply, and a high demand for stones over one carat, this blazing beauty can demand the highest price per carat of any coloured gemstone.

Once thought to have held the power of life due to its likeness in colour to blood, rubies are still a highly coveted gemstone, signifying wealth, success, love and passion.

Ruby is the red variety of corundum, a crystalline form of aluminium oxide. Colourless in its purest state, corundum is ‘allochromatic’ – meaning it relies on trace impurities to influence its colour. For ruby, chromic oxide replaces some of the alumina in the crystal structure.

The amount of chromium present determines the strength of ruby’s red, while the presence of other elements, such as iron, influence tone and hue.

Other than the orange-red through to strong purplish red stones, which are called ruby, gem quality corundum is known as sapphire, prefixed by its colour. Paler reds or pinks are thus appropriately named pink sapphire.

The finest, rarest quality rich-red Burmese rubies come from the Mogok (old source) or Mong Hsu (new source) mines.

"The dominance of treated stones on the market means that consumers should trust their local Gemmologist or registered NCJV valuer to ascertain their true identity"

These chromium-rich crystals form in a white marble and because they contain no iron, the result is vivid pinky-red stones that can show fluorescence in sunlight, adding to their intensity and value.

In comparison, the rubies of Cambodia and Thailand originate in iron-rich basalt and are typically darker. These stones have an orange-red colouring because the iron impurities diminish the vividness caused by chromium.

In the past eight years, Mozambique – a location of recent ruby discoveries – have become a dominant source of commercial quantities of varying quality and colour.

African supplies have traditionally produced darker stones, however the new mines produce colours that bridge the gap between those from the classic sources of Myanmar (low iron, strong fluorescence) and Thailand/Cambodia (high iron, low fluorescence) suiting a range of different markets.

A ruby’s value is determined not only by colour, but its clarity, cut and carat size. Consumers must be aware of the multitude of treatments and synthetics.

Heat treatment is common practice as it parallels what can happen in nature. The heating process removes silk inclusions, enhancing clarity and richness of colour.

Although it does affect the price, if heat treatment does not add anything artificial to the stone, it is an accepted treatment amongst gemmologists.

In more recent years glass has been used to fill fractures in rubies, but while the stone is made more attractive, the glass fill can dramatically decrease its durability. Sometimes called ‘composite rubies’ – but more accurately ‘glass fracture-filled natural ruby’ – the nature of such stones should be explained and priced accordingly.

Interpretation of colour may be subjective, but there is no denying the beauty of an intense red ruby.

The dominance of treated stones on the market however, means that consumers should trust their local Gemmologist or Registered NCJV valuer to ascertain their true identity.












ABOUT THE AUTHOR
Stacey Lim

Contributor • Registered GAA Gemmologist & Valuer


Stacey Lim FGAA BA Design, is a qualified gemmologist and gemmology teacher/assistant. She is a jewellery designer, marketing manager and passionate communicator on gemmology. For information on gemstones, visit: gem.org.au









Sunday, 23 September, 2018 05:09pm
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