This glass-like opal, usually found associated with volcanic rocks, has characteristics different from other 'solid' types of opal. It tends to have a lower water content, and very small amounts of contaminating elements. It is often found in cavities in volcanic rocks coating the upper surfaces, and even forming small stalactites.

Examples from Rocky Bridge Creek in New South Wales have been found where the base of the cavity is filled with white opal, clearly deposited from an aqueous carrier, while hyalite coats the top of the cavity Flörke et al (R0215) have studied hyalite, and concluded that it has a glass-like atomic arrangement, and has probably been deposited from water above the critical point.

Hyalite-type opal also occurs in situations where temperatures and pressures were unlikely to have been so high. For example, at the Dome Rock copper mine in South Australia, hyalite-like opal may be found coating, as a final deposit, secondary copper minerals such as conichalcite. This material has not been studied, so it cannot be said yet whether or not it is identical in its properties with hyalite from volcanic rocks.