opals are VERY delicate stones. just like kitsunegrl said, easy to scratch & crack.
here's some info on opals:
A species of quartz, opal is one of the few gemstones that are sedimentary in origin. Millions of years ago, after ancient seas receded, silica-laden sediment was deposited around shorelines. Erosion made much of this silica into a solution that filled cracks in rocks, clay and fossils. Layers upon layers of silica jell were added to each other over millions of years and became precious opals. The stones still contain 6 to 10 percent water, a remnant of ancient seas. Because they have high water content, opals should be protected from heat and strong light in order to prevent them from drying out and cracking.
Opals are delicate, but well worth the care. Their most significant weakness has to do with the water content. If an opal is allowed to dry, it will crack and craze. In most cases, they do not need any special care while stored. However, if you live in a very dry climate, or keep them 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.
Because of their water content, opals are also highly sensitive to sudden changes in temperature. Opals do not mind being hot or cold, it is the rate of change that damages them. You need to avoid situations like going from a warm house to the winter's cold. Simply wearing an opal under clothing will protect them. Also, do not store opals near a heat source, an open window, etc., where they can be exposed to sudden temperature changes.
Being somewhat soft, they scratch easily. Realize that a large component of dust is quartz at 7 in hardness. At 5.5 to 6 in hardness, simply wiping the dust off an opal will gradually reduce its polish. The solution is to clean your opals using a soft cloth or brush, a mild detergent, and room temperature water. Then rinse the jewels to remove any residue.
Prevention is the best solution to scratching and chipping. Opals are best suited for earrings, brooches, and pendants. These jewels receive little contact with harder objects, compared to what a finger ring experiences. If you do get an opal ring, choose a setting that protects it from coming in contact with other objects.
Understand that, if you wear an opal ring on a regular basis, it will require occasional repolishing. Reserving your ring for special occasions will greatly reduce the risk of damage.
Make sure you remove your ring before physical activities like gardening and sports. Also, do not immerse the gem in liquid chemical solutions, like dishwater. Opals are porous and absorb liquids.