Though opals are found around the world, Australia is the biggest source . Precious opal is a hydrous silicon dioxide material, that flashes iridescent colors when it is seen from different angles, or when the is moved or when the light source is moved. This is also know by the term Koroit-Opal.( Boulder Opals)

Certain opals display quite different colors once viewed from different directions, or when the Koroit-opal is turned, or when the light source is moved around. This phenomenon is known by the term "play of color" or opalescence. The color in many Opals is truly exceptional and very pleasing to then eye. The wide range of color in a particular Opal is determined by the diameter and spacing of the silica spheres. Certain colors of an Opal can change depending upon the angle of light incidence, so when an Opal is rotated, its color can change or disappear completely.

Boulder Opals or Koroit-opals is a term for opal mining this is not a actual city or suburb.

Boulder Opals exhibiting such colors, are termed precious and are very valuable, those without it are termed common and have little or no value what so ever. The desirability of opal is based upon color Intensity, and diversity, and uniformity, pattern and ability to be seen from any angle. Most precious Koroit opals will show or reflect a number of colors. The only other major amorphous gemstone is amber. Good quality opals are transparent, not milky. The huge number of colors in Opals are caused by the regular array of silica spheres ,diffracting white light, and breaking the light into the colors of the light spectrum.

Opal is a soft stone, as well as a delicate stone. It is usually milky and translucent. Opal is a hardened silica gel containing 5-20% water. Boulder Opals have a rating a 5.5 to 6.5 on the hardness scale. Opal is amorphous, meaning, it has no crystal structure. Opals may be somewhat porous, in which case it is dangerous to immerse it in liquids other than water. Opal is made from small silica spheres arranged in a regular pattern. The type of silica in Opals is chemically similar to quartz, but Opal silica contains water within the mineral structure. Precious Opal generally contains about 6-10% water.

Boulder opals are found in several places in Australia (including finds like Big Ben). Queensland has the highest area of Koroit or boulder opal fields. 70 per cent of the worlds quality opals come from this little Australian settlement. Opals were discovered 1915.

Opals have been considered a mystical precious stone for hundreds of years. It is said to help the wearer's psychic powers. Opals are supposed to further enhance the positive characteristics for those people born under the zodiac sign of Cancer. Koroit-opals is very chemically similar to quartz, Rubies are said to change color according to the health of the wearer,Opals are also softer type of gemstone, compared to that of diamonds. This may have contributed to the perception of opals being "bad luck".