The birthstone for October and the state gemstone of Nevada, the opal is a semiprecious crystalline stone prized for its "fire" or color patterns caused by tiny pieces of silica embedded within the stone. Opals can be found in the southwest United States and the Australian outback. Here's a guide on how to find and care for them.
Look for arid inland sedimentary deposits with a lot of silica. Almost all of the opals mined in Australia come from sedimentary rock. Opals also can be found in igneous (volcanic) rock, however.
Visit an opal farm where you can sift (noodle) through tailings; Nevada offers several such places, such as Rainbow Ridge. You also can start your own mine, but even though opal is found at depths of less than 100 feet, this requires more work.
Seek rocks with smooth, rounded shapes on the outside, glasslike substances within the rock or colors different from the surrounding rock. Opals form when liquefied silica percolates through sediment, condenses to a gel, then dries into gemstone as the water is drawn away.
Examine the rock thoroughly. A small discontinuity could be the tip of a large opal.
Search for cracks to break the rock open; opals are often found near such fractures. Cracks make chiseling easier, but give the rock a good whack anyway to break away the surrounding stone from the opal.
Cut the rest of the rock away from the opal with a diamond band saw or let a lapidary do it for you.