Rough opal specimen from Virgin Valley, NV - pic courtesy of Wikipedia
Opal is classified as a mineraloid because of its amorphous character. It’s a hydrated form of silica and may occur in fissures of almost any kind of rock, but most commonly in limonite, sandstone, rhyolite, marl and basalt. Opal is the national gemstone of Australia.
Opal from Australia - pic courtesy of Wikipedia
The internal structure of opal makes it diffract light. It can take on many colors, but usually is clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown and black. Black opals are rarest. White and green opals are the most common.
Opal mined near Queensland Australia - pic courtesy of Wikipedia
Opal was rare and very valuable years ago. In Europe it was a gem prized by royalty. Today the state of South Australia accounts for about 80% of the world’s opal.
Showing the "fire" in opal - pic courtesy of Wikipedia
Opal pendant from Ethiopia - pic courtesy of Wikipedia
In the Middle Ages opal was considered a stone that could bring good luck. It could also confer the power of invisibility, if wrapped in a fresh bay leaf and held in the hand.
In Sir Walter Scott’s novel, Anne of Geierstein, opal acquired the reputation that it held supernatural powers. In the novel, the Baroness of Amheim wears it as a talisman. When holy water drops on the necklace, the opal turns into a colorless stone and the Baroness dies shortly thereafter. Due to the popularity of this novel people began to associate opal with bad luck and death. This had a great impact on the opal market. Within a year of publishing Scott’s novel in April 1829, the sale of opals in Europe dropped 50% and they remained low for the next twenty years.