The most desired opals are prized for their iridescent colors and often thought of as being in the gem family. However, while not crystalline like most other gemstones but rather a hardened silica gel, opals are an all-time favorite for both art and jewelry - old and new. Also, since opals are a silica gel, this means they are composed of 5-10% water.  Opal has a Mohs hardness of 6 making it a 'softer' stone for jewelry.
There are several types of opals and these are usually categorized either by their base type or value. Some of these are:
Iridescent precious black opal has iridescent flashes of color in a black bases,
The iridescence in the precious opals is caused by light being diffracted by the arrangement of the tiny silica spheres. The larger the spheres - the broader the range of colors. This iridescence can include every color of the light spectrum. Needless to say, if shopping, you will find a wide variety of quality in each of these types.
crystal opal has color play within a transparent or translucent base,
common 'white' opal has minimal color within an opaque or milky base with no iridescence and
fire opal is colorful and transparent without the color play.
Boulder opal may be another name you hear. This is not a 'pure' opal but rather where the opal is displayed within the host rock by either filling the cracks and cavities in sedimentary rock or the veins in igneous rocks. These 'opals' can be cut - including the host rock - and then polished to look every bit as beautify as any of the iridescent opals you might see. 
With opals most often thought of as being iridescent, it should not be overlooked that opals also come in solid colors like pink, blue, honey and many other hues. These opals are often faceted and prong set while the iridescent ones are left smooth (cabochon) and set in a bezel for strength. It should be noted than many of the opals being brought from Africa are being faceted just like a traditional gemstone for jewelry. Most of the African opals we've seen have had a more orange looking base, rather than the prominent blues of the Australian opals.
The more desirable opals come from Australia with the regions of Coober Pedy and Lightening Ridge being the most recognized names. However, opals are found in other parts of the world as well. To name a few:
- United States
- Czech Republic
When buying opals buy quality stones that are clear and not milky. Also, caution should be exercised since there are opal imitations, i.e., Slocum stone (glass) and lab opals (circa 1973).
Another caution when buying opals is to look for layering. You'll find these opals may be comprised of two to three layers, doublet and triplet, respectively. In the case of doublet opal items, they are comprised of two layers where a thin opal layer is on top of a potch (common non-iridescent opal) base to enhance the appearance. Triplets will have a crystal or clear top layer placed over the opal and base layers. To check for layering, look along the side of the opal for a line indicating a distinctive separation of the top and bottom. You do not want to pay top price for a doublet
The fact that opals contain water will influence the way you care for them since they may crack or dry out. Caring for your opals is important and we recommend that a professional jeweler be contacted and methods be verified before attempting to clean any opal. 
 Smithsonian Handbook, Gemstones
- If you live in a very dry climate, you can keep them in an air-tight plastic bag with a piece of damp fabric.
- Wear your opal under clothing if you are going to be experiencing a significant change in temperature.
- Do clean your opals (doublets and triplets) by using a soft cloth or brush, a mild soap and rinse with room temperature water to remove residue. (See Don'ts below)
- Do wear opal jewelry that is less likely to be hit or scratched, e.g., rings have a lot of exposure to damage. Earrings, pins and pendants are more suited to opals. If you do buy a ring, a bezel setting will help protect the opal.
- Do have the surface re-polished if the stone becomes dull. This will help to retain the beauty.
- Do not store opals in oil or glycerin - it is difficult to clean off.
- Do not submit the opal to sudden changes of temperature, i.e., warm home to freezing outside.
- Do not store near a heat source, open window, in sunlight, or any other place they can be exposed to sudden temperature changes.
- Don't damage your opal's surface by wiping the surface as gritty-dirt will leave scratches.
- Don't soak doublets or triplets in water as this may dissolve the glue.
- Don't wear your opal during physical activities like exercising, sports, gardening, cleaning, etc.
- Don't immerse your opal in any chemical / cleaning solution or expose it to hand sanitizers since opals are porous and will absorb the liquid.
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