Mexican Opals are more transparent, with less defined colouring and arranged in much larger patches. We also get fire Opals from Mexico which are of one colour, the deep red "fire-like" stones being the best although they vary in colour to a warm yellow.
The History of Opals - The Eye stone
In the fourteenth century the Opal was known as the Ophthalmius, or Eye Stone, because it was believed to sharpen and strengthen the eyesight. It was also believed that its flashes of coloured fire were especially helpful 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 History of Opals - Unlucky?
The idea of opals being unlucky stones had its origin in the misfortunes that befell Anne of Geierstein, or The Maiden of the Mist (1829) in Sir Walter Scott's novel, her principal jewel consisting of a large Opal. In the East opals are regarded as a sacred stone which contains the Spirit of Truth.
The History of Opals - Ancient Greece
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. Any misuse brought ill-luck in love (which probably accounts for its being unlucky when used in an engagement ring) and disappointment and misfortune in all enterprises.
The History of Opals - Ancient Rome
According to the Roman author Pliny, as an illustration of the high value of the topaz, Nonnius, a Roman Senator, endured outlawry and exile at the hands of Marcus Antonius rather than part with an Opal he possessed.