History: |
The Romans first popularised Opal about 100 BC and gave it the name opalus (to see a change of colour). They prized Opal above all other precious stones and believed the holder to be charmed with good fortune. The only European source of this rare and prized gemstone prior to the sixteenth century was the volcanic deposits of Czechoslovakia (previously part of Hungary). Later the Spaniards brought back fine examples of Aztec Opal from the New World. Beautiful Opals have featured in many of the Crown Jewels of Europe and they remain popular with heads of state and the rich and famous. |
Occurrence: |
Australia produces approximately 95% of the world's precious opal. Other deposits (Volcanic) occur in North, Central and South America, and Indonesia. |
Formation: |
During the Cretaceous period (65 - 140 million years ago) the vast desert region of central Australia was an inland sea. Through the mid Tertiary period (approximately 30 million years ago) ground water rich in silica permeated down through sedimentary rocks accumulating in the apertures created by faults and dissolved objects. Solidification occurred and the silica spheres were arranged into a close packed structure. |
Structure: |
Opal is a hydrated silica (SiO2.nH2O) with a water content within the mineral structure. Precious Opal contains 6% - 10% water and consists of small silica spheres arranged in an orderly three dimensional structure. |
Colours: |
Silica spheres of similar size must be aligned into a regular three dimensional structure for light to be diffracted into a spectral colour. Smaller spheres result in the diffraction of the Blue end of the spectrum and larger spheres the Red |
