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More model informationChemically, opal is a hydrated form of silica [SiO₂•n(H₂O)]. Since it is amorphous an lacks a regular crystaline structure, it is not technially a mineral, but a mineraloid. In terms of physical properties, it is similar to quartz, but is slightly softer, a lower density and different lustre. Like quartz it has a conchoidal fracture and no cleavage. Note that only a small proportion of this specimen is opal - the remainder is a matrix of hydrous iron oxides (goethite and others). Also note that the colours of opal are highly variable, and even though this specimen displays some gem quality blues, most opal has little colour (if any) and is often just a creamy off-white.
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