mineral

Blue Topaz (specifically London Blue shade)

Topaz - Al2SiO4(F,OH)2

Hardness: 8 on the Mohs scale; Color: Deep teal-blue; Luster: Vitreous (glassy); Crystal Structure: Orthorhombic; Cleavage: Perfect basal; Specific Gravity: 3.4–3.6.

Hardness: 8 on the Mohs scale
Color: Deep teal-blue
Luster: Vitreous (glassy)

Identified March 11, 2026More mineral →

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Physical properties

Hardness: 8 on the Mohs scale; Color: Deep teal-blue; Luster: Vitreous (glassy); Crystal Structure: Orthorhombic; Cleavage: Perfect basal; Specific Gravity: 3.4–3.6.

Formation & geological history

Formed in acidic igneous rocks like granite and rhyolite, often in pegmatite veins or cavaties. While natural blue topaz exists, most 'London Blue' specimens start as colorless topaz that is irradiated and heat-treated to achieve the deep color.

Uses & applications

Primarily used in jewelry (rings, pendants, earrings) due to its high hardness and clarity. It is also a popular collector's gemstone and the birthstone for December.

Geological facts

London Blue is the deepest and most valuable of the three standard treated blue topaz shades. Despite being common in the market, it takes roughly two years of cooling/aging after irradiation before the stone is safe for humans to wear.

Field identification & locations

In the field, it is identified by its high hardness, inability to be scratched by quartz, and heavy feel for its size (high specific gravity). It is commonly sourced from Brazil, Sri Lanka, and Nigeria.

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