mineral

Blue Topaz (specifically London Blue or Swiss Blue based on color saturation)

Al2SiO4(F,OH)2 (Aluminum Silicate Fluoride Hydroxide)

Hardness: 8 on Mohs scale. Color: Highly saturated blue (treated). Luster: Vitreous (glassy). Crystal structure: Orthorhombic. Cleavage: Perfect basal. Specific gravity: 3.49–3.57.

Hardness: 8 on Mohs scale
Color: Highly saturated blue (treated)
Luster: Vitreous (glassy)

Identified March 11, 2026More mineral →

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Physical properties

Hardness: 8 on Mohs scale. Color: Highly saturated blue (treated). Luster: Vitreous (glassy). Crystal structure: Orthorhombic. Cleavage: Perfect basal. Specific gravity: 3.49–3.57.

Formation & geological history

Formed in igneous environments, typically in pegmatites and granitic rocks as late-stage crystals from fluorine-bearing vapors. While natural blue topaz exists, deep blue gems are typically produced by electron or neutron irradiation of colorless topaz followed by heat treatment.

Uses & applications

Primary use is in fine jewelry (rings, earrings, necklaces) as a durable and affordable blue gemstone. Also used in some industrial applications requiring thermal stability, though synthetic sapphire is more common for those roles.

Geological facts

Topaz is the state gemstone of Texas and the birthstone for November (yellow) or December (blue). The intense 'London Blue' color shown in the image is a result of high-energy irradiation and can take up to two years to reach a safe radioactivity level for sale.

Field identification & locations

Identify by its high hardness (it will scratch quartz) and high refractive index. In the field, look for well-defined prismatic crystals in alluvial deposits or within cavities of granitic host rocks. Common locations include Brazil, Sri Lanka, and Nigeria.

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