Topaz is an aluminium fluorosilicate, a hard stone at 8 on the Mohs scale but with one perfect cleavage that makes it liable to split, and yellow to golden-brown is its classic historical colour, imperial topaz is the prized pinkish-golden grade. A caution runs through its history: the ancient name topazos, said by Pliny to come from an island in the Red Sea, was applied to yellow-green stones that were probably peridot, not topaz, so much topaz in classical and medieval sources is another mineral, and conversely many yellow gems called topaz in old jewellery are citrine quartz. True topaz was securely distinguished only in the modern era. It is a November birthstone. Much blue topaz in today's market is colourless material turned blue by irradiation. Its metaphysical associations, solar and abundance-themed for the yellow stones, are modern and colour-led.
Medieval and Renaissance lapidaries repeat a persistent claim that a genuine topaz would grow visibly dimmer in the presence of poison and recover its brilliance once the danger passed, a belief with no basis in the stone's actual optical or chemical properties but repeated across centuries of gem-lore as a mark of the stone's protective virtue. The confusion of topaz with other yellow stones, noted above from Pliny, persisted into the modern era: the Braganza, a colourless 1,680-carat stone long displayed among the Portuguese crown jewels and believed for generations to be the largest diamond ever found, was eventually identified as topaz, a correction that embarrassed collectors well into the nineteenth century.