Fire opal and diamond ring, 1920. Fire opals are an unusual variety of opal, found primarily in Mexico. Popularised in the 19^th century, fire opals only occasionally show the rainbow iridescence associated with most opals, but more than make up for it with their distinctive and highly attractive body colours, ranging from a glowing orange to a deep cherry red. The rectangular fire opal at the centre of this ring – a rare cut for a fire opal - shows a glowing orange with a delicate Belle Epoque mount millegrain-set with single-cut diamonds – an alluring blend of fire and ice.