Tourmaline
complex borosilicate · Trigonal · also called schorl, elbaite, watermelon tourmaline
Tourmaline is instantly recognisable once you have seen the striated, triangular prisms. Black schorl is common in granite and pegmatite; the colourful varieties collectors chase are far rarer and come from a handful of pegmatite districts.
Properties
- Mohs hardness
- 7–7.5
- Streak
- white
- Lustre
- vitreous
- Colour
- Black (schorl) accounts for the overwhelming majority of tourmaline found. Pink, green, and colour-zoned crystals come almost entirely from pegmatites.
- Cleavage
- Effectively no cleavage; it fractures instead.
- Fracture
- Conchoidal to uneven
- Specific gravity
- 3–3.3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm tourmaline
- Crystals have a rounded-triangular cross-section and heavy lengthwise striations — like a bundle of drinking straws.
- Hardness 7–7.5, so it scratches glass easily.
- Warming a crystal makes it develop a static charge that picks up ash and paper scraps.
What tourmaline gets confused with
Each of these is settled by one test. Run that test first.
Hornblende has two cleavages at about 120° and is softer; tourmaline has no cleavage.
Apatite is much softer and its prisms are hexagonal, not triangular.
Where it is found
Granite pegmatites and metamorphic rocks; black schorl is widespread in granite country.
Is it worth anything?
Schorl is worth very little. Colour-zoned and Paraíba-type tourmalines are among the more valuable coloured gemstones.