Lussatite

Lussatite Is described in Dana's Textbook of Mineralogy 4th ed. page 473. It was first described by the Frenchman E. Mallard, One of the earliest mineralogists to examine common opals using the optical microscope. As it appeared to be a distinct mineral, he gave it the name 'lussatite.' His conclusions were published in a paper entitled Sur la Lussatite, nouvelle varieté minérale cristallisée de silice, published in 1890.

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the International Mineralogical Association has so far rejected his results, but has not solved the nature of the tiny spheres that we know are responsible for the display of color in precious opal. Light passing through or being diffracted by orderly spaced uniformly sized spheres of SiO₂ is proven, but the nature of the spheres has remained illusive.

Some suppose they are cristobalite, others that they are tridymite, or some combination, but no one is sure. The 1996 study by Cady et. Al. indicates perhaps Mallard was right all along.