Opal is the official birthstone for the month of October, as well as considered the mystic birthstone for the month of April.
Formula: SiO2 · nH2O
Although it is still (2007) regarded as a valid mineral species for historical reasons, Opal is not a true mineral in the accepted sense of the word as it is either composed of cristobalite and/or tridymite or composed of amorphous silica.
Opal derives its name from the Roman word opalus, a Latinized version of the older Sanskrit upala, meaning "precious stone". Opal is a hardened silica gel, and usually contains 5-7% water in submicroscopic pores. Its structure varies from essentially amorphous to partially crystalline. Precious opal is the least crystalline form of the mineral, consisting of a regular arrangement of tiny, transparent, silica spheres with water in the intervening pores.
Opal is very widespread. In its pure form, it is essentially colorless. The vast majority is common "potch" opal in opaque, dull yellows and reds imparted by iron oxides, or black from manganese oxides and organic Carbon. It is especially abundant in areas near hot spring activity and, as the siliceous skeletons of diatoms, radiolarians, and sponges, opal constitutes important parts of many sedimentary accumulations such as diatomaceous earth. It is commonly found as fossilized wood where it preserves the wood's external appearance and cellular structure.
Precious Opals can form only in undisturbed space within another rock that is capable of holding a clean solution of silica from which water is slowly removed over a long period of time-perhaps thousands of years.
Opal is considered an October birthstone. Some people think it’s unlucky for anyone born in another month to wear an opal. But that particular superstition comes from a novel written in the 1800s (Anne of Geierstein by Sir Walter Scott), and not from any ancient belief or experience. In fact, throughout most of history, opal has been regarded as the luckiest and most magical of all gems because it can show all colors. Once, it was thought to have the power to preserve the life and color of blond hair.
Many cultures have credited opal with supernatural origins and powers. Arabic legends say it falls from the heavens in flashes of lightning. The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them against disease. Europeans have long considered the gem a symbol of hope, purity, and truth.
Wearing opal as a jewelry stone is a relatively modern practice. However, in ancient times, people wore this stone for various reasons. Many considered opal to be beneficial to the eye and wore it to cure eye diseases. Some even believed it could render the carrier invisible. Supposedly, carrying an opal wrapped in a fresh bay leaf would keep others from seeing you. This superstition earned opal the popular designation of patronus furum, Latin for “patron of thieves.”
Some believe the darkness and depth of the black opal can hold and release power for magicians in their arcane workings. Of course, whether they use this power for good or bad is up to their discretion and has no bearing on the stone itself.
Until late in the 19th century, the primary source of precious opal, including that used by the Romans, was in present-day Slovakia, where opal occurred in andesite. Today, the chief producer of precious opal is Australia, where it was discovered in 1887, with deposits in South Australia, Queensland, and New South Wales. In Australia, opal is found in sedimentary rock such as sandstone and ironstone. The Lightning Ridge field in New South Wales, Australia, produces the rare and prized black opal, with a very dark grey or blue-black body-color and a superb color play. Until the discovery of this field in 1903, black opal was virtually unknown and is still the rarest form of opal.