What is an Opal?
The composition of Opal is variable. Chemically, it is defined as a hydrated form of Silica (SiO2nH2O), with the water content of the stone being from 1% to 20%. Precious gem opal usually contains approximately 6% to 10% water. The hardness of the Opal ranges from 5.5 to 5.6 on Moh's scale of hardness, or midway between the softest and hardest gemstones.
Opal is originally formed from a solution of silica in water, hydrated Silica. This Silica was concentrated by evaporation to a state of gel, which then filled cracks and cavities in sandstone, ironstone or basalt, and further evaporation of the water caused the gel to set into "Solid" Opal.
The colour of precious Opal is caused by the diffraction and interference of light waves through the stacked layers of silica spheres which make up the Opal. If these spheres which make up the Opal are evenly sized (approximatedly 0.00025mm each) in diameter, and are stacked in an orderly manner, the colour range produced by diffraction and refraction of the light waves can cover the full spectrum (or colours of the rainbow).
Should the silica spheres be uneven in size, and irregularly stacked, no vivid spectrum colours will be produced, and the Opal is then called the "Common Opal" or "Potch", with little or no commercial value. Common opal, as the name implies, is readily available at low cost, but Precious or Gem Opal is very hard to find and because of its rarity, it is eagerly sought after for jewellery and investment purposes.
