Ruby the red variety of corundum occurs as bright-red, red, purplish- or brownish-red, and deep pinkish-red hues, coloured by chromium. It is the birthstone for those born in July.
The toughness, stability and hardness of both natural and synthetic ruby, coupled with its red fluorescence means it is not just desirable in jewellery, but also used in watchmaking, lasers and medical instruments. Diamond is the only gemstone harder than ruby.
The colouring element chromium in ruby, can cause fissures and cracks, so few crystals large enough to be classified as gem quality ruby are found naturally. This rarity, along with is desirable characteristics make ruby a precious and expensive purchase.
Three important factors of quality affect the value of the gemstone: colour, clarity and quality of cut. The highest value stones have a good rich colour, are free from inclusions and have a bright, lively appearance.
Commercial quantities of ruby are found in numerous locations including Myanmar, Pakistan, Afghanistan, Thailand, Sri Lanka, Tanzania and Vietnam. These locations have fairly distinctive inclusions to aid identification, but the inclusions are frequently seen in gemstones from other areas and may occur in stones from other localities such as Australia, Kenya, Namibia, Madagascar, India, USA, Russia, China and Nepal.
|Locality||Typical Inclusion and Features|
|Myanmar (Mogok district)||Commonly contain short, fine rutile needle inclusions known as silk. A variety of mineral inclusions are also seen. Inclusions may be well-formed, or corroded rounded crystals. Intersecting twining planes often seen.|
|Pakistan||Similar to those from Myanmar in colour but clarity is often poor, generally stones are cut as cabochons.|
|Afghanistan||Often contain blue patches, as well as calcite, mica and rutile inclusions. Usually found in small sizes.|
|Thailand||Naturally darker, brownish-red in colour. Often have irregularly-shaped fluid inclusion with dark crystals at their centres. Heat treated stones may be similar in colour to stones from Myanmar. Weak fluorescence is seen due to iron content. Intersecting twining planes often seen.|
|Sri Lanka||Stones contain long, coarse, rutile needles, biotite mica, pyrite, metamict zircon grains with tension hales, pronounced hexagonal colour zoning, elongate negative crystals or cavities containing fluid and/or crystals. Red and pink in colour.|
|Tanzania||Stones found in Longido are often of fine colour, found in a bright green chrome zoisite rock. Due to clarity stones are cut as cabochons. In the Umba valley facetable material is found showing twinning planes and apatite crystals. Morogoro is where star material is found.|
|Vietnam||Fine colour, good clarity, similar to Myanmar rubes. Most material contains blue patches, which can be removed or modified by heat treatment.|
A sixteenth-century writer wrote of the July birthstone that it: "gave control of the passions, drove out evil thoughts, secured possessions to their rightful owner, reconciled quarrels, brought peace and concord and also preserved bodily strength and health." In the past many red stones with a good hardness and lustre, including ruby, spinel and garnet were referred to as ruby. As gemmological knowledge improves, the differences between these materials is more easily identified, although incorrect identification does still occur. Materials with a similar appearance today include red glass, red tourmaline, almandine pyrope garnet, red spinel, red garnet-topped doublet and corundum-corundum doublet. ■
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Cover image ruby with rutile inclusions. Image courtesy of Pat Daly.