Colors are formed by the diffraction of light by silica-spheres. The more neatly the various sizes of spheres are aligned, the more color the opal will show. Potch is opal where spheres are of irregular size, shape or in irregular alignment. In 1964, an Australian scientist, Dr. J. Sanders, made scanning possible with electron photo-micrographs. He found that opal is composed of minute particles of silica in closely packed spherical aggregates of uniform size varying in diameter from 150 nanometers to 300 nanometers. If spheres are packed tightly together and of uniform size, we have gem-quality opal.

The colorplay of Opals comes in a wide variety of patterns. The most beautiful and sought-after pattern must be the "Harlequin" Pattern. It's rare and demands top-prices for a well-defined show of pattern. For various Harlequin Patterns, please see the book by Barrie O'Leary - "A Field Guide To AUSTRALIAN OPALS". "Pinfire", "Chinese Writing" and "Rolling Flashfire" are a few of the many more well-known patterns.

The value of an opal depends highly on the range of colors in the colorplay. The highest value is given to Red multicolor, sliding down the scale to Orange, Green, Blue and Purple. Red is the rarest color of all. Of all opal found, only two to three percent shows a reasonable amount of red.

Therefore the rarity of the colours (most common to least common) is as follows: blue, green, yellow, orange, and red. Opals which display red can also display all the other colours of the spectrum. Therefore the possible combinations of colours in an opal can be seen as: blue only, blue-green, blue-green-yellow, blue-green-yellow-orange, and finally the full spectrum of blue-green-yellow-orange-red. For this reason, the presence of red in an opal can greatly add to its value, since it is somewhat of a rarity. It can also be deduced that the light diffraction in the voids is greatest when the sphere size is greatest. Therefore, generally speaking, red is usually the brightest opal color and blue is duller.

Potch, also known as common opal, is any type of opal which does not display any color. In this case, the silica spheres may be absent, too small, or too irregularly arranged to produce colour. (Opal which does display colour is known as precious opal). Potch is virtually worthless, although it often serves as an excellent dark backing for black opals which normally have a thin segment of precious opal naturally formed on a potch backing.