Blue cobalt spinel is a rare and highly prized variety of spinel distinguished by its vivid, pure blue color caused by trace amounts of cobalt in its crystal structure.

Here’s a detailed description:

• Color: Intense, rich cobalt blue — often compared to the finest blue sapphires but with a slightly more electric or “neon” tone. The color is usually uniform, without zoning.

• Transparency: Transparent to translucent, with top specimens being highly transparent and eye-clean.

• Luster: Vitreous (glassy) and bright, giving it a lively sparkle when cut.

• Hardness: 8 on the Mohs scale, making it durable for jewelry.

• Refractive Index: ~1.718, contributing to its brilliance.

• Crystal System: Isometric (cubic), typically forming octahedral crystals.

• Origin: The most famous sources are Sri Lanka, Vietnam, and Tanzania. Cobalt spinels from Luc Yen, Vietnam, are especially renowned for their exceptional color saturation.

• Rarity: Very rare, especially in larger sizes (over 1 carat) with pure cobalt-blue color and no grayish tones.

• Treatment: Usually untreated — the color is naturally caused by cobalt, unlike sapphire which is often heat-treated.

Value Factors:

• Color purity and intensity are the top value drivers — the most vivid, saturated cobalt blues can command prices rivaling fine sapphires.

• Clean clarity and precision cutting further increase desirability.