Opal, common Opal. SiO2 . nH2O. Opal is a hydrated amorphous form of silica, and so it is classed as a mineraloid. This common opal variety is from the Norseman opal mine in Western Australia, and is locally known as gold lace opalite. The variety is described as being a yellow brown opal with black dendritic inclusions. This specimen is as the variety description, and was collected by Jim Nicholls, co-founder of our club while living near Norseman.