Ruby
Ruby- the name given to red, gem-quality corundum- is one of the best gem stones for jewellery settings. Rubies may be any shade of red, from pinkish to purplish, or brownish red, depending on the chromium and iron content of the stone. Frequent twinning of the crystals makes the material liable to fracture, yet ruby is a tough mineral, second only to diamond in hardness. Crystal prisms are hexagonal with tapering or flat ends. As the crystals grow they form new layers, and depending on the geological conditions and minerals present, colour variations, called zoning, occur.
Occurrence
Worldwide in igneous and metamorphic rocks, or as waterworn pebbles in alluvial deposits. The finest stones come from Burma; those from Thailand, the main source, are brownish red; Afghanistan, Pakistan, and Vietnam produce bright red stones; those from India, North Carolina (USA), Russia, Australia, and Norway are dark, sometimes even opaque.
Remark
In 1902, a Frenchman, Auguste Verneuil, produced a synthetic ruby crystal by exposing powered aluminium oxide and colouring material to the flame of a blowtorch.
| Crystal structure | Trigonal |
| Composition | Aluminium oxide |
| Hardness | 9 |
| SG | 4.00 |
| Rl | 1.76-1.77 |
| DR | 0.008 |
| Lustre | Vitreous |
