Zultanite (gem-quality color-change diaspore) has a specific gravity of approximately 3.30 to 3.39, depending on trace-element content. It is heavier than quartz (2.65) and tourmaline (3.06), comparable to topaz (3.49–3.57), and lighter than chrysoberyl/alexandrite (3.71). A 1-carat Zultanite occupies slightly more volume than a 1-carat alexandrite.

The measurement

Specific gravity is the ratio of a substance’s density to the density of water at 4°C. For diaspore, published values range from 3.30 to 3.39, with most gem-quality material from the İlbir Mountains testing near the high end of that range due to iron substitution in the aluminum site.

Volume vs weight

Because Zultanite is less dense than alexandrite (the classical color-change benchmark), a Zultanite cut to the same carat weight will appear visibly larger. A 2-carat Zultanite is roughly the size of a 2.2-carat alexandrite at equivalent proportions.

Diagnostic use

Specific gravity is a primary identification tool. Diaspore’s value sits in a narrow band that excludes most lookalikes: at 3.30–3.39 it is too heavy for green tourmaline (~3.06) and too light for chrysoberyl (3.71), helping confirm species when refractive-index measurement alone is ambiguous.