mineraloid
Green Opal or Yellow Opal
Hydrated Silica (SiO2·nH2O)
Hardness: 5.5-6.5 on Mohs scale. Color: Pale green to yellowish-green. Luster: Vitreous to waxy. Crystal Structure: Amorphous. Specific Gravity: 1.9-2.3. Cleavage: None, conchoidal fracture.
- Hardness
- 5
- Color
- Pale green to yellowish-green
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 5.5-6.5 on Mohs scale. Color: Pale green to yellowish-green. Luster: Vitreous to waxy. Crystal Structure: Amorphous. Specific Gravity: 1.9-2.3. Cleavage: None, conchoidal fracture.
Formation & geological history
Formed from the precipitation of silica-rich water in fissures and cavities of volcanic or sedimentary rocks at relatively low temperatures. Common in various geological ages where there is groundwater activity in volcanic terrains.
Uses & applications
Used primarily in jewelry making for cabochons and beads, as well as for decorative carvings, lapidary arts, and metaphysical collecting.
Geological facts
Unlike precious opal, common opal does not show 'play of color.' The color is often due to iron inclusions or chlorite in the case of green specimens. It is technically a mineraloid rather than a mineral because it lacks a crystalline structure.
Field identification & locations
Identify by its waxy luster, smooth conchoidal (shell-like) fracture, and relatively light weight. Commonly found in Madagascar, Tanzania, Brazil, and the USA. It will not scratch glass easily compared to quartz.
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