Ring silicates

trigonal (+)

Relief - moderate

Colour - brown, blue, pink

RI - 1.610-1.655

Birefringence - 0.021-0.026

Interference colours - 2nd-3rd order

Extinction - straight



brown, colour-zoned tourmaline


Aplite, Megilligar Rocks



colour zoned tourmaline with quartz


St Austell Granite



colour zoned tourmaline with quartz; the image above under crossed-polars


St Austell Granite



Above; brown tourmaline with epitaxial growth of acicular blue tourmaline (left) and the same field of view rotated through 90° to show the extreme pleochroism (right)

< the same crystals under crossed polars


St Austell Granite

brown primary tourmal with epitaxial growth of blue secondary tourmaline in PPL (left) and XPL (right)

Luxullianite, St Austell Granite


Tourmaline 'suns' in PPL (left) and XPL (right), the latter showing the straight extinction cross

Luxullianite, St Austell Granite

prismatic crystal of tourmaline in PPL (left) and XPL (right).

St Austell Granite


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