Opal

Opal: a compound of hydrated silica (SiO₂·nH₂O), colloidal in nature, amorphous, semi-amorphous, or microcrystalline (due to the presence of small amounts of Cristobalite and Tridymite), formed in aggregates of tiny spherical particles. It contains water in varying amounts between 1% and 21%. Colors range from colorless, white, yellow, orange, red, brown, green, blue, purple, gray, to black. When displaying the characteristic “play of color,” it is highly valued.

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