Gem library
Black tourmaline
Schorl, the protective stone par excellence
Black tourmaline, or schorl, accounts on its own for more than 95 % of all tourmaline on the planet. Its striated prismatic crystals are a pegmatite classic — and it has electrical properties few minerals share.
- Hardness (Mohs)
- 7 – 7.5
- Density
- 3.10 – 3.25
- Crystal system
- Trigonal
- Refractive index
- 1.635 – 1.675
- Cleavage
- Very imperfect
Mineralogical identity
- Family
- Silicates — cyclosilicates, tourmaline group
- Species
- Schorl
- Variety
- Black tourmaline
- Chemical formula
- NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄
- Crystal system
- Trigonal (rhombohedral)
- Classification
- Collector's mineral
- Rarity
- Very common
- Indicative price
- €0.20 – 3/ct
Etymology
Tourmaline comes from the Sinhalese turamali, «stone of mixed colours». Schorl is an old Saxon miners' term for vein waste.
Physical & optical properties
- Hardness (Mohs)
- 7 – 7.5
- Density
- 3.10 – 3.25
- Refractive index
- 1.635 – 1.675
- Birefringence
- 0.017 – 0.025
- Cleavage
- Very imperfect
- Fracture
- Splintery to conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Opaque
- UV fluorescence
- None
- Pleochroism
- None
- Colours
- black, bluish black
Formation, geology & history
Piezoelectricity and pyroelectricity
Tourmaline is both piezoelectric — compressed, it generates a charge — and pyroelectric: heated or cooled, it spontaneously develops an electrical polarity, positive at one end, negative at the other. Eighteenth-century Dutch merchants put this to practical use: heated in the ashes, a tourmaline attracts ash and straw, hence its nickname aschentrekker, the ash-drawer. The property follows from the absence of a centre of symmetry in its structure: a tourmaline crystal is not identical at its two ends, which its differing crystal terminations also reveal — a phenomenon called hemimorphism.
One mineral, a very accommodating structure
The tourmaline group is chemically among the most complex in the mineral kingdom: its formula accepts sodium, calcium, lithium, iron, magnesium, manganese, aluminium, chromium and vanadium at different sites. Schorl is the iron pole, opaque black; elbaite, rich in lithium, gives the coloured tourmalines; dravite, magnesian, the browns; uvite and liddicoatite complete the family. The crystals are typically prisms of rounded triangular cross-section, deeply striated lengthwise — a very recognisable morphological signature in granites.
Use and market
Without gem value, black tourmaline is abundant and very cheap: it is found as rough crystals, points, pebbles, beads and inclusions in quartz (tourmalinated quartz, much appreciated). Its hardness of 7-7.5 and the absence of true cleavage make it robust, suited to everyday jewellery. Prized collector pieces are clean, terminated crystals on a matrix of feldspar and mica — notably those from Pakistan and Madagascar. It is also used in powder form in industrial applications that exploit its pyroelectricity.
How to recognise it
- Deep lengthwise striationsThe tourmaline prism is marked with sharp grooves parallel to its axis. No other common black mineral has that fluted look.
- Rounded triangular cross-sectionIn section the crystal traces a triangle with blunted corners — a major diagnostic trait.
- Hardness 7-7.5It scratches glass. Obsidian (5) and onyx (7) behave differently, and hematite is far denser.
Imitations, treatments & confusions
- Black glassBubbles, mould seams, no striations, immediate warmth in the hand.
- Obsidian and onyxA frequent confusion rather than fraud. Obsidian is a soft, sharp volcanic glass, onyx a plain chalcedony; neither has the striated prismatic structure.
- Filled black resinLight, warm, scratched by a needle, dull-sounding when knocked.
Deposits & origins
Brazil, Madagascar, Pakistan, Namibia, Norway, United States (California, Maine), China, Austria, France (Brittany, Massif Central — in granites and pegmatites)
Care & precautions
- Warm soapy water and a soft brush: robust, with no true cleavage and no porosity.
- Ultrasonic-safe on sound stones; avoid on fissured crystals or pieces on matrix.
- Insensitive to sunlight. Avoid prolonged salt on pieces with a mica matrix.
- Fine crystal points break on impact: store collection pieces apart.
Traditions & crystal healing
- Chakras
- Root
- Zodiac signs
- Capricorn · Scorpio
- Element
- Earth
The most cited protective stone in all contemporary crystal healing: it is said to form a shield against negative influences and waves, and to «discharge» other stones. It is traditionally placed near doors, screens and passageways. Note that these uses are recent and rest on no documented ancient tradition.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Black tourmaline or obsidian?
Tourmaline is a striated prismatic crystal of hardness 7.5; obsidian is an amorphous volcanic glass of hardness 5 with a sharp fracture. Tourmaline scratches glass, obsidian does not.
Is black tourmaline really anti-wave protection?
No scientific study shows it, and the mineral has no measurable effect on domestic electromagnetic radiation. Its reputation comes from contemporary crystal healing, not from ancient tradition.
Why is it so cheap?
Because it is extremely abundant: schorl accounts for more than 95 % of all tourmaline. Only the coloured varieties (elbaites) reach gem prices.