Opal

What is Opal?

Opal is hydrated silica with a disordered or poorly crystalline structure, valued for play-of-colour and natural pattern.

Mineralogy and structure

The reference formula is SiO₂·nH₂O. Opal is classified with the mineraloid and its crystal system is not applicable to this non-crystalline material. Natural specimens can vary in composition, habit and degree of crystallinity, so the locality and observed form remain important.

Colour and appearance

Typical colours include white, colourless, yellow, orange, brown, blue and black. Lustre, transparency, inclusions and surface condition should be described for the individual specimen rather than inferred from the name alone.

Formation and localities

Opal forms in or is associated with silica-rich groundwater in cavities, weathering profiles and sedimentary environments. Notable collecting regions include Australia, Ethiopia, Mexico, Slovakia, Indonesia and the United States. A precise locality is more useful than a broad country label when it is documented.

Collecting notes

Opal is useful in a reference collection because its appearance connects mineral chemistry with geological setting. Store specimens dry, protected from impact and clearly labelled with the confirmed material, locality and any treatment history.