Mexican Fire Agate a variety of chalcedony is a semi-precious natural gemstone found only in certain areas of northern Mexico and the southwestern United States.There is no actual object inside the stone.
Boulder Opal: Opal takes its name from the Latin word Upala meaning precious stone. The Roman scholar Pliny described opal as ring having the fire of carbuncle (a deep red garnet) & the brilliance of amethyst with the green color of emerald.
Gem quality opal from Australia. SEM Image of the opal. Compare near-gem quality opal and gem-quality fire opal x magnification) from (dry) Lake Tecopa, Inyo County, California, USA; SEM images courtesy of Ma Chi