Formula: SiO₂ · nH₂O

Species:

Colour: colourless, white, creme-coloured

Lustre: Vitreous

Hardness: 5½ – 6½

Specific Gravity: 2.0 – 2.2

A variety of: Opal. Opal-AN is a term for amorphous opal with a glass-like structure. The “A” in the name stands for amorphous; the subscript “N” is to imply its structure is network-like similar to silica glass; however, it still contains about 3-8% water, both as molecular water and bound in silanole (SiOH) groups. This form of opal occurs as hyalite in volcanic and pegmatite environments where the silica deposits from the gas phase and forms globular and irregular crusts. Because of its homogeneous internal structure, it does not show opalescence and is transparent if pure.