Rubies are made up of the mineral corundum. Corundum is aluminum oxide ( Al2O3). A ruby forms when there is a small impurity of chromic oxide in the corundum (an aluminium oxide in which some of the aluminum ions have been substituted by chromium). This substitution process is called an isomorphous replacement. Chromium and vanadium, another metal constituent of rubies, provide the fiery red color of the crystal. If the stone formed in the corundum is not red, it is a sapphire, although the name is commonly associated with only blue stones.

Example of Corundum

Another name for Corundum is Emery. On Mohs Scale of Hardness, Corundum is a 9, just under diamonds (10).

Ruby crystals form at high temperatures, between 620 and 670°C. They appear to be the features of distal hydrothermal reactions with marble. "They are formed mainly in high-grade metamorphic environments where hydrothermal fluids meet limestone," (Waltham, 1999, p. 144). When there are rutile inclusions present in cabochons, there is a star effect.

Star Effect

Rubies can also be created in a laboratory, using two different methods: Flame-Fusion and Flux-Growth.

The cheapest method of producing synthetic rubies is the flame-fusion process. The chemicals are melted and dripped onto a boule. The melt then crystallizes within a matter of hours. The result is an unnatural, glassy stone with curved growth plates. Instead of inclusions, flame-fusion rubies have tiny gas bubbles. They cost $1-4 per carat. Such a ruby is often used in costume jewelry.

Trillion Cut Ruby

The flux-growth method involves dissolving the chemicals into a molten mixture, called a flux. The gem is inserted into the flux, and it crystallizes under controlled-pressure conditions. It takes up to six months for the crystal to form. The growth planes are straight, making the crystal structure look more like a natural ruby. Often flux-growth rubies have inclusions which only skilled gemologists can decipher from natural ruby inclusions. Flux-growth rubies can range in cost from $100-500 per carat. Because these rubies look so real and are much more affordable, many people buy them... but no geologist ever would!

Literature Cited