Mineral: Opal

Composition: SiO₂.nH₂O

Crystal system: Amorphous; massive, banded and colliform forms

5.19.1
moss opal
Marlborough central Qld Australia

Properties:

• Variable lustre; vitreous, resinous, silky
• Variable color; white, cream, red, brown, also with an iridescent play of colors
• White streak
• Knife barely scratches or not at all (Mohs Hardness 5.5-6.5)
• Below average density
• No cleavage
• Smooth (conchoidal) fracture
• Not ferromagnetic
• Does not react with hydrochloric acid (HCl)

Occurrence:

Opal is essentially amorphous although specialised characterisation shows it consists of close packed silica spheres with variable water content. Opal is deposited from low temperature silica-rich waters around hot springs and in fractures and cavities in volcanic and sedimentary rocks, or it replaces other materials such as wood . Extensive layers of diatomite interbedded with other sedimentaryrocks are composed of diatoms, ie, the minute opaline skeletons of algae-like plants.

5.19.2 moss opal Marlborough central Qld Australia	5.19.3 boulder opal Winton north west Qld Australia

Colored and iridescent forms of opal are used for jewellery and ornamental purposes. Diatomite is used as an industrial filtering agent and in manufacturing.

5.19.4
boulder opal
Winton north west Qld Australia