Opals are by far and away my most favourite gem…. they can show an incredible display of colour. Because I like them so much I have acquired connections to obtain some of the finest available.
Opals are a hydrated amorphous form of silica and their water content can range from about 3 to 20%… but is usually between 5 and 10%.
Comprised of tiny spheres of silica, opals bend light as it enters them, resulting in play-of-color. Different size spheres produce different colours.  Depending on the conditions in which it formed, opal may be transparent, translucent or opaque.
Care must be taken with opals… but just use common sense.
1) At 5.5 to 6.5 on the moh’s scale, opals are softer than some other gems.  But. Bear in mind that a steel knife is only 5 on the scale… so just some caution is necessary.
2) Their high concentration of water requires them not to be subjected to rapid temperature changes.
3) The fact that they absorb water can cause problems if care is not practiced. Placed in water they can absorb more and lose their colour display. This will reappear in a day or so as they dry out…. but if there are impurities in the water the opal may take some of the impurities into itself…. but dropping your opal into a wash basin, glass of wine or cup of coffee, or getting caught in the rain are not really a concern: absorption is not immediate and requires more time than a quick dip to take effect. Just best not to take chances: do not immerse in coloured or dirty liquids for extended periods.
4) Avoid sonic cleaners or steamers.