At the Morenci copper mine Southeast of Phoenix different semi-precious stones are found, which can be processed into beautiful cabochons: malachite, azurite, chrysocolla, and the so-called "gem silica". "gem silica" is a material that is colored turquoise to blue-green. This chalcedony receives its interesting color by finely dispersed particles of chrysocolla. Chrysocolla is a difficult material for cabbing and a very soft material, while "gem silica" shows a good grinding and polishing behavior.
"Gem Silica"is found at other sites too - for example, at the Lily mine in Peru.
Jeffrey Anderson is describing this location on his website dwarve´s earth treasures.