The origin of the term “opal” is unclear – it may be from the Roman word opalus, the Sanskrit úpala, the Greek opallios, or any of many other possibilities. In this way, the opal extends its mystery beyond the well known color play into the name of the stone itself.
Natural opal has a lot of variety, including: white opal, black opal, jelly opal, boulder opal, opal matrix, fire opal, harlequin opal, crustal opal, Andean opal, Ethiopian opal, and common opal. White and black opal are most commonly found and these typically display the gorgeous color play which from “the diffraction of light off tiny, closely packed silica spheres inside the stone.”  The silica gel inside the stone is 5-30% water. Most of the world’s natural opals today come from Australia – particularly Lightning Ridge (black opal) and Cooper Pedy (white opal).
Because opal is soft (5.5-6.5 on the Mohs scale), it is often sold as doublets or triplets, which provide color enhancement as well as some protection for the stone. There are also synthetic man-made opals, like Gilson opal and opalite. Be sure to ask your seller for information about any stone you purchase.
Opal can be easily damaged by pressure and impact. Opal is also sensitive to acids and alkalis because of its porous nature, which also makes it vulnerable to perfumes, soaps and detergents. Jewelry should always be removed before washing or applying lotions and other similar products.
If an opal is allowed to dry, it will crack and craze. In most cases, opals do not need any special care while stored. However, if you live in a very dry climate, or keep opals in a dehumidified room, some precautions are necessary. Keeping them in a tight plastic bag, with a damp piece of cotton or fabric will prevent dehydration.
Opals do not mind being hot or cold, it is the rate of change that damages them. You need to avoid exposing the stone to a sudden change in temperature, like from a warm house to the winter’s cold. Simply wearing an opal under clothing will protect it.
Clean your opal with warm or room temperature soap and water. Avoid wearing the stone where it will get rough treatment.
According to lore, opal helps relieve issues with eyesight and was believed to obscure its wearer with a thick fog. 
There are other metaphysical properties ascribed to this multi-colored gem:
- Makes it easier to handle changes in life
- Protects the wearer from harm
- Supports renewal and fidelity in love
- Intensifies emotions and intuition
- Helps to express your true self
Chakras: links the root and crown
 The Jeweler’s Directory of Gemstones by Judith Crowe, p.92
 A Lapidary of Sacred Stones by Claude Lecouteux, p.244