Sapphire

Hardness: 9 on the Mohs scale. Color: Deep blue to blackish-blue in this specimen. Luster: Vitreous (glassy). Crystal structure: Trigonal/Hexagonal. Cleavage: None (has parting). Specific Gravity: 3.95-4.03.

Formed in aluminum-rich metamorphic rocks (like marble or schist) or igneous rocks (like syenites) under high pressure and temperature. The user's coordinates in Western France (near Angers/Maine-et-Loire) are in the Armorican Massif, which contains metamorphic zones where corundum can occur, though gem-quality sapphire is more often sourced from alluvial deposits in volcanic regions.

Primary use is in high-end jewelry as a gemstone. Industrial uses include watch crystals, high-durability windows, and abrasives due to extreme hardness.

Sapphire is the second hardest natural mineral after diamond. A 'Black Sapphire' like the one pictured often owes its dark color to high concentrations of iron and titanium, or internal inclusions.

In the field, it is identified by its inability to be scratched by a steel file or quartz, and its high density. Note: The location 47.5°N, 0.8°W is characterized by the Armorican Massif's metamorphic rocks, making the presence of corundum mineralogically possible, though faceted gems are rarely found in the wild there.