opal is a hydrated amorphous form of silica. because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classify as minerals.
amethyst is a violet variety of quartz. the name comes from a greek word, a refered to the belief that the stone protected its owner from drunkenness. the ancient greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.
emerald is a gemstone and a type of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. emerald in antiquity were mined in Egypt at locations on Mount Smaragdus since 1500 BCE.
