Technically speaking, turquoise is the hydrous basic aluminum phosphate plus copper. It is found in most of the Southwestern copper producing states, with Nevada and Arizona in the lead, followed by Colorado and New Mexico. Some turquoise comes from mines worked specifically for the gemstone; some comes from copper mines as a by-product. But many think that because a turquoise stone came from the Bisbee stake mine in Arizona, for instance, that it is superior quality. Each mine has varying grades of turquoise and only a small percentage of any mines production is of high grade quality. There are four major evaluations for determining the quality of turquoise; Density, hardness, color and matrix.

Density is the weight of a certain substance as related to its volume. A box of foam rubber, for instance, will have less density than a similar box of iron. The more dense a turquoise stone is, the higher polish it will take; with less space between the molecules of the stone, the better its resistance to fracturing.

Hardness is a factor in evaluating and gemstone. On a scale 1 to 10, high grade turquoise should be between 5.7 and 7. Some turquoise is chalky soft and is therefore regarded as a lower grade.

Color, while a matter of personal preference as to the range from sky blue to grass green does not affect the intrinsic value of the stone as long as it has density, hardness and a well bonded matrix. However, deep blue turquoise stones are the most sought after among buyers.

The the American Southwest, where turquoise is the hallmark of the regional lifestyle, matrix is important. Elsewhere in the world, turquoise with no matrix at all and in a paler sky blue color is highly regarded.

The matrix is defined as the type of rock or mineral that is mixed with the turquoise. It appears as spider webbing or veining on the face of the turquoise stone. Its color can range from black and brown to lavender. If one of the combining minerals of the matrix is of low density, the stone will fracture easily in cutting or wearing. There's one other consideration in evaluating turquoise. That of course, is the size of the stone. Since most turquoise comes in small veins or nuggets, the larger pieces are rare. Therefore a turquoise stone the size of a coin purse, for instance, may cost twice as much per a carat as a turquoise stone of equal quality but half the size.