Rubies-Composition

Rubies belong to the corundum family, which also includes all sapphires. While sapphires can come in many colors, rubies are defined specifically by their rich red hue. The presence of chromium gives rubies their unmistakable color, ranging from pinkish red to deep blood red.

Chemical Composition and Crystal Structure

Rubies are composed of aluminum oxide (Al₂O₃), with chromium as the trace element responsible for their red coloration. The exact shade of red depends on the chromium concentration and the presence of other elements like iron. Rubies have a trigonal (hexagonal) crystal system and typically form as prismatic or tabular crystals. Their structure, combined with their exceptional hardness (second only to diamonds at 9 on the Mohs scale), makes rubies one of the most durable gemstones available.

Despite their hardness, rubies may contain natural inclusions formed during their intense geological growth process. These inclusions, sometimes called “silk” when composed of fine rutile needles, are often viewed as signatures of a ruby’s natural origin.

Formation of Rubies

Rubies form under conditions of intense heat and pressure within the Earth’s crust, typically in metamorphic rocks like marble or in basalt-associated deposits. In marble-hosted rubies, low iron content allows for the formation of the highly saturated, pure red colors prized by connoisseurs. Basalt-hosted rubies, often containing more iron, can display darker tones.

The ruby’s vibrant color, rarity, and strength all result from these challenging formation environments, which require specific combinations of pressure, temperature, and chemistry.