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At Lake Tecopa, in California, white play-of-color opals are found in vesicles of a volcanic ash layer from the Huckleberry Ridge Tuff super-eruption (2.1 Ma). They show characteristic traits of opal-AG by X-ray diffraction and scanning electron microscopy (silica spheres of ~330 nm). These properties are not typical for volcanic opals, and are usually associated with opals formed in a sedimentary environment, such as opal-AG from Australia. The conditions under which opal was formed at Lake Tecopa were determined by oxygen and hydrogen isotopic analyses and give a better understanding of the formation of opal in general.

american mineralogist

Insight on gem opal formation in volcanic ash deposits from a supereruption: A case study through oxygen and hydrogen isotopic composition of opals from Lake Tecopa, California, U.S.A