Hydrated silica mineraloid
Opal is a mineraloid, not an ordinary crystalline species. Fire opal concerns warm bodycolor; play-of-color is a separate feature.
Keep each chapter’s scientific identity: a family, species, variety, trade label, rock and fossil are different kinds of names.
Mineraloid
Opal is hydrated silica, often with a milky, translucent or colorful appearance. Precious opal can show shifting play-of-color, while common opal can be attractive without that optical display.
Robert M. Lavinsky · CC BY-SA 3.0 · Display crop
Mineraloid variety
Pink common opal is usually appreciated for its soft body color rather than shifting rainbow flashes. Opaque and translucent pieces can have very different visual depth.
Géry PARENT · CC BY-SA 3.0 · Display crop
Mineraloid variety
Fire opal is yellow to orange or red opal. Some examples show play-of-color, but many transparent orange gems are prized without that moving effect.
DonGuennie (G-Empire The World Of Gems) · CC BY-SA 4.0 · Display crop