Opal
Opal is a mineraloid / tectosilicate mineral prized by collectors for its exceptional color range, with known Chinese sources.
About Opal
Opal is classified as a mineraloid / tectosilicate mineral in the silica group and has the chemical formula SiO2·nH2O. It crystallizes in the amorphous system and is one of the most visually varied minerals in the collector market. Its combination of structural character and global distribution make it a recognized species in both systematic and aesthetic collections.
Identification & care
Opal typically forms amorphous; nodular, botryoidal, vein-filling, replacing fossils; no crystal form. Its color range is broad, including colorless, white, black, orange, red, yellow, green, blue, and play-of-color in precious opal. The luster is waxy, vitreous, resinous, the streak is white, and specimens range from transparent to opaque. The fracture is conchoidal, which is one of its key identifying features.
Collector context
How it forms
In terms of geology, Opal forms in low-temperature silica precipitation from groundwater; weathering profiles; volcanic hydrothermal; replacement of fossils and bones. It is commonly found in association with chalcedony, quartz, calcite, limonite.
Classic Chinese localities
Opal is widely represented across Chinese provinces, including Fujian, Gansu, Anhui, Guangdong.
Why collectors care
Collectors pursue Opal for its patterns, color depth, and polish response rather than for pattern character. Good material has a surface that polishes cleanly, a visual character that holds up in direct light, and enough size to anchor a display on its own. Chinese sources, where present, supply much of the material currently cut and sold as decorative pieces.
What affects value
Value in Opal is assessed, in typical order of weight, against: (1) source locality; (2) size; (3) pattern and visual character; (4) color depth and distribution; (5) polish response and surface finish; (6) piece integrity (absence of major cracks or chips). Uniqueness of pattern and verified source region add significantly to decorative pieces.
Naming history
The name Opal has a specific etymological and historical context — see Mindat's reference entry for provenance details. We have retained naming data at the record level; published prose is paraphrased from factual fields rather than copied from source.