What is a variety in mineralogy ?

Variety : definition

A variety is a mineral that has a color, or a particular or original morphology that has led to it being given a name other than the usual name : emerald is a variety of beryl, amethyst a variety of quartz, etc...

A variety is not distinguished by a significant change in chemical composition (the usual stoichiometric formula remains the same), nor by a change in crystalline structure, so it is not another mineral.

The appearance of a variety generally results from very small variations in the chemical composition of the mineral : amethyst is a violet variety of quartz (presence of iron), emerald a green variety of beryl (presence of traces of chromium or vanadium), the ruby and sapphire of the corundum varieties (respective presence of traces of chromium and titanium). More rarely, it is the form that decided the individualization of a variety : chalcotrichite (acicular variety of cuprite), adularia (variety of potassium feldspar with unusual crystals), etc...
Amethyst from Adrar Tirecht Bou Oudi, Morocco
Emerald from Panjshir, Afghanistan
Sapphire from Ratnapura, Sri Lanka

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