Hypersthene

Hypersthene is an orthopyroxene in the enstatite-ferrosilite series, where magnesium and iron substitute freely for one another. Traditionally the name referred to compositions with roughly 30 to 50 percent of the ferrosilite end member. In 1988 the International Mineralogical Association discredited hypersthene as a valid species name and folded it into enstatite, so the correct modern name for most of this material is ferroan enstatite. The old trade name survives because cutters, dealers, and older references all still use it.

Hypersthene crystallizes at high temperature in mafic and ultramafic igneous rocks, including gabbro, norite, and pyroxenite, and in high-grade metamorphic rocks of the granulite facies. The large schiller-rich masses sold as specimens come from anorthosite complexes, where orthopyroxene occurs as coarse cumulate grains alongside plagioclase feldspar. Labrador, Ontario, Norway, and parts of the Indian shield all host such bodies.

The characteristic bronze-to-black iridescent flash, called schiller, comes from exsolution lamellae of ilmenite or magnetite. As the pyroxene slowly cooled, minor titanium and iron oxides unmixed along preferred crystallographic planes, forming thin ordered layers inside each crystal. Light reflects off those layers to produce the coppery metallic sheen that moves across the surface when the stone is rotated.

Hypersthene is dark brown, greenish black, or nearly black, with a vitreous to submetallic luster and a specific gravity between 3.4 and 3.9 that feels noticeably heavy in hand. Hardness is 5 to 6, so it will scratch glass but is scratched by quartz. The streak is grayish white to pale brown.

The single most useful diagnostic feature is the schiller. When a polished surface is tilted under a direct light, a bright bronze, copper, or sometimes dark red flash sweeps across the stone in a narrow band. The reflection stays sharp and metallic, not rainbow colored, and it comes from inside the crystal rather than the surface.

Two cleavage directions meeting at nearly 90 degrees mark it as a pyroxene and distinguish it from amphiboles, which cleave at about 120 and 60 degrees. Crystal faces, when present, are prismatic and orthorhombic in habit.