Back to Materials Opal Opal is a peculiar stone. It has a water content that can be upto 20% of the structure. Again as with so many minerals silicon dioxide is a large part of the chemical composition. Rather than being a crystal, it can more properly be classed as a gel, due to the water content, although what is actually occuring inside is millions of crystal structures held in this gel. Light is easily diffracted in opals, and this is why opals have been much sought after. The myriad colours that can be viewed (all the colours of the light spectrum) can produce a truly remarkable semi-precious gemstone. However many of the stones available tend to diffract light in small wavelength bands, rather than the whole light spectrum. We use opal chips from australia, and opal beads from china in our designs, both of which are a white variety of opal. Opal has a specific gravity of approximately 2.15-2.9 (2.15 times the weight of water by volume, and its hardness value is between 5.5 and 6.5 on the mohs scale. Opal is used to celebrate the 34th wedding anniversary.