The Opal is the most beautiful and mysterious of all gems, containing as it does all the colours of the rainbow, which flash and glow in sparks and minute flames as the light plays over the surface, causing it to be aptly described as combining in itself the beauties of all the other stones. It is a species of soft quartz, and the wonderful play of colour is believed to be caused by extremely minute particles of air enclosed in its fissures.

The finest variety is known as Harlequin Opal, in which the colourings are distributed in numerous very small flakes throughout the stone. Mexican Opals are more transparent, with the colouring less definite, and arranged in much larger patches; from Mexico we also get fire Opals which are of one colour, the deep red "fire-like" stones being the best, though they vary in colour to a warm yellow.

Opals are very soft when first taken from the mine, but harden by exposure. Great care is necessary in cutting and also in setting them, as they are very brittle and liable to chip.

In the fourteenth century the Opal was known as the Ophthalmius, or Eye Stone, because it was believed to sharpen and strengthen the eyesight; also that its flashes of coloured fire were especially efficacious in arresting the glance of envy. In India, the passing of an Opal across the brow is believed to clear the brain and strengthen the memory.

The idea of its being an unlucky stone had its origin in the misfortunes that befell Anne of Geierstein in Sir Walter Scott's novel, her principal jewel consisting of a large Opal; they are not, in reality, more unlucky than other stones, though being a Libra gem and essentially a pledge of friendship, they are not fortunate for any one having Venus afflicted in their horoscope. In the East it is regarded as a sacred stone which contains the Spirit of Truth, and in Ancient Greece the Opal was supposed to possess the power of giving foresight and the light of prophecy to its owner, provided it was not used for selfish ends; its misuse bringing ill-luck in love (which probably accounts for its being unlucky when used in an engagement ring) and disappointment and misfortune in all enterprises.

Pliny tells, as an illustration of its high value, that Nonnius, a Roman Senator, endured outlawry and exile at the hands of Marcus Antonius rather than part with an Opal he possessed.

All Opals are very sensitive to atmospheric conditions, varying in brilliancy according to the temperature, their colouring being at its best when worn and kept warm and dry. This sensitiveness was believed by the ancients to make them susceptible to influences of an occult nature, so that when the colour of an Opal was bright and lively it indicated success and good fortune to enterprises or travel, and when dull and lifeless it warned of failure and disappointments. It also indicated to its wearer whether it was favourable or otherwise, being full of colour and brilliancy when in sympathy with its owner's interests and lacking in colour and lustre when adverse in its influence.