Zultanite (gem-quality color-change diaspore) measures 6.5 to 7 on the Mohs hardness scale, comparable to peridot and below quartz. This places it in the moderately hard range — durable enough for pendants, earrings, and occasional-wear rings, but vulnerable to scratching from quartz dust (5–7 Mohs) and harder gemstones in jewelry boxes.

The measurement

The Mohs hardness scale, devised by Friedrich Mohs in 1812, ranks minerals by their resistance to scratching. Diaspore tests at 6.5 to 7, depending on crystallographic direction (the mineral exhibits perfect cleavage on the {010} plane, so hardness is direction-dependent at the boundary).

What this means for jewelry

At 6.5–7 Mohs, Zultanite is harder than opal (5.5–6.5), tanzanite (6–7), and apatite (5), and similar to peridot (6.5–7), garnet (6.5–7.5), and tourmaline (7–7.5). It is softer than quartz (7), topaz (8), spinel (8), corundum (sapphire/ruby, 9), chrysoberyl including alexandrite (8.5), and diamond (10).

Practical implications

Airborne quartz dust scratches anything below 7 Mohs over time. Zultanite jewelry — particularly rings worn daily — will accumulate microscopic surface wear. The gem is best suited to pendants, earrings, and occasional-wear rings than wedding bands or daily-wear pieces.

Cleavage matters more than hardness

Diaspore has perfect cleavage on the {010} plane — meaning a sharp impact along that plane can split the stone cleanly. This is a more critical durability concern than the Mohs number. Master cutters orient the table facet to minimize cleavage exposure, but the gem remains susceptible to point impacts.