Opal, SiO2 . nH2O is not a mineral, because it has a water content, and so it is classed as a mineraloid, It is classed as a hydrated amorphous form of silica, with a water content usually in the range of 6% to 10%, but it may have as much as 20%. Opal is composed of microscopic spheres of silicon dioxide molecules, which are deposited in closely packed planes. If the planes are very precise and regular. precious Opal is produced, however, most of the time slight discrepancies in how the spheres are deposited, result in "rough Opal". This specimen is rough opal from Madagascar , with just a hint of regular deposition.