You are looking at an Australian Opal skull Brooch. It's 2.3 long, from front to back. It can also be used as a pendant. Technically, opal is non-crystalline silica (in a rigid gel form) containing approximately 5% to 9% water. Non-crystalline silica is similar to quartz and sand. Opal is not only one of the world's most beautiful precious gemstones; it is also one of the worlds rarest. Opals rarity has made it a desirable and at times divisive part of many cultures over thousands of years. Opal is from the Greek "Opallos" meaning to "see a change of colour. Each Australian opal is unique and the different varieties offer a spectacular choice of colour, shape and size. When rotated, the opal displays a constantly changing interplay of fiery colours. The finest quality stones can command prices similar to the finest emeralds, rubies and diamonds.