Opal is a silica based material with a very high water content. Their colour play (or fire) comes through light refracting off the minute silica particles, which have an irregular structure. To keep opals happy do not let them sit in the sun, or dry out, which might lead to them cracking, crazing or dulling. An occasional soak in water is a good idea - mine always have extra sparkle after I have done this.
Most opals are feminine and watery in their nature, yet is would be a mistake to see them as gentle or soft! The watery nature makes them all generally useful at dealing with emotional conditions, or in drawing out deep or long-held restrictive and paralysing beliefs about one's Self. They are the "tears of the Earth" and, like tears, help us to confront and release things that may prevent forward progress, or that have built up within to such a level that this has resulted in physical disease or discomfort. The common opals are gentler in their approach than the precious opals, but these too (through the intensity of the full-spectrum of Light they can hold and channel) have huge potential for confrontation with, release, and subsequent clearance of anything that does not truly serve us and our life purpose. The newer transparent / opaque opals have come now to awaken us to the full expression of who we are, and to the reality and possibility of joy in our lives.