TR (Türkçe)
Mineral family
Tourmaline & the layered silicates
A grouping by structural habit rather than by chemistry. Tourmaline, the micas and kyanite all build in layers or blades, and that architecture explains their most striking behaviour: a mica you can peel with a fingernail, and a kyanite blade that is soft along its length and hard across it.
At a glance
Tourmaline is the most chemically variable common gem. The micas and kyanite share its habit of growing in layers, which is what makes them soft in one direction and not another.
- Keynote
- Crystals that grow in sheets and blades
- Chemistry
- borosilicates & micas
- Crystal systems
- Trigonal, monoclinic & triclinic
- Mohs range
- 2.5 to 7.5
- Stones here
- 4 in the catalogue
- Care rule
- Wildly different hardness across the group. Check the number before you wear it.
- Watch for
- Dyed and heated tourmaline; lepidolite blended into fantasy trade names.
Themes
boundary
the layered self
two strengths
reconciliation
charge
what grows in sheets
What holds the family together
Tourmaline is a boron silicate with a formula so accommodating that mineralogists describe it as a rubbish bin for elements. Iron gives black schorl, lithium and manganese give the pink and green elbaite, magnesium gives brown dravite. Because the chemistry can change while a crystal grows, a single tourmaline can be pink at one end and green at the other, or pink in the core and green in the rind, which is watermelon tourmaline.
Tourmaline is also pyroelectric and piezoelectric. Heat one end and it develops opposite electrical charges at each end, which is why Dutch traders in the eighteenth century used it to pull ash out of a pipe and called it aschentrekker, ash-drawer.
The micas, including lepidolite, are sheet silicates that cleave into flexible plates in one direction. Lepidolite is a lithium mica at hardness 2.5, soft enough to mark with a fingernail.
Kyanite is the group's oddity: hardness 4.5 along the blade and 7 across it, a difference so marked that its old name, disthene, means two strengths.
How it forms
Tourmaline forms in pegmatites and in some metamorphic rocks, and the great gem sources are Brazil, Afghanistan, Pakistan, Nigeria, Madagascar and Maine and California in the United States. The Californian mines at Himalaya and Tourmaline Queen supplied a large trade with China in the early twentieth century, driven by the Dowager Empress Cixi's preference for pink tourmaline; when the Qing dynasty fell in 1911 the market collapsed with it.
Paraiba tourmaline, coloured by copper and discovered in Brazil in 1989, is among the most expensive gems by weight in the world.
Lepidolite occurs in lithium-rich pegmatites, often with tourmaline, and is mined commercially as a lithium ore.
Kyanite forms in aluminium-rich metamorphic rocks under high pressure, and geologists use its presence to read the pressure and temperature history of a region.
The stones of this family
- Black tourmaline (complex borosilicate (schorl), Mohs 7.5). Pyroelectric, so it attracts ash when warmed, which is why the Dutch called it aschentrekker. The single most-sold protective stone in modern practice.
- Watermelon tourmaline (elbaite, zoned pink & green, Mohs 7.5). One crystal that changed its chemistry as it grew, leaving a pink core in a green rind. Slices show it best, and good ones are not cheap.
- Lepidolite (lithium mica, Mohs 2.5). A lithium mica, soft enough to mark with a fingernail, and the source of a persistent modern claim about mood that its chemistry does not support.
- Kyanite (Al₂SiO₅, Mohs 4.5 / 7). Two hardnesses in one crystal, soft along the blade and hard across it. Used as a compass needle in early Europe, and the mineralogist's favourite oddity.
Trade names and lookalikes
Black tourmaline is rarely faked because it is cheap, but heat and irradiation are used on coloured tourmaline to improve colour, and this is common and generally stable.
Watermelon tourmaline slices are imitated with resin, glass and assembled doublets. Genuine well-zoned slices are not cheap, and the colour boundary in a real one is a crystallographic zone rather than a printed line.
Paraiba is the family's fraud hotspot, since the price differential between copper-bearing Paraiba and ordinary blue-green tourmaline is enormous, and laboratory testing for copper content is the only reliable check.
Lepidolite is often sold blended with quartz under invented trade names. It is also the subject of a persistent claim that its lithium content affects mood, which the chemistry does not support.
Kyanite is honest by default, though blue material is sometimes confused with sapphire in rough form.
Care that follows from the structure
Read the hardness before you decide anything. Black tourmaline at 7.5 will take a pocket indefinitely. Lepidolite at 2.5 will not survive a keyring and should be kept in a pouch or on a shelf.
Kyanite is the interesting case. It has perfect cleavage in one direction and very different hardness along and across the blade, so it can be scratched by a fingernail on one face and resist a steel file on another. Set it protectively or keep it as a specimen.
Tourmaline should not be steam cleaned or put in an ultrasonic bath, particularly the included and heavily coloured material, because internal fractures are common and thermal shock can propagate them.
Lepidolite should not be soaked, since water penetrates between the mica sheets and can cause it to delaminate over time.
Düşündüren sorular
- Where am I strong in one direction and easily split in another?
- What in me changed partway through growing, and left a visible boundary?
- Which of my defences is a genuine limit and which is only a habit?
Stones in this family
Frequently asked questions
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