Black tourmaline is the iron-rich end of the tourmaline family, and it is the only common tourmaline that comes out black. The 
mineralogical name is schorl. It grows in long prisms with fine lines running the length of each face, and those lines are the quickest way to tell a genuine piece. They are growth lines in the crystal. At seven to seven and a half on the hardness scale it is one of the tougher stones you can keep.

Tourmaline is piezoelectric, which is to say it builds up an electrical charge when it is squeezed, and pyroelectric, so it also does that when it is warmed. Rub a piece in your hands on a cold morning and it will do it. 

The name came to Europe through Dutch traders working out of Sri Lanka, from the Sinhalese word turmali. The black material was already familiar in Britain long before that, though for the wrong reason: Cornish miners hit it constantly while working tin lodes and knew it as schorl, and it was treated as waste. 

Deposits of quality black tourmaline now come out of Brazil, Madagascar, Pakistan, Afghanistan and Nigeria, and it usually turns up in pegmatite, which is the coarse-grained rock it grew in.

Traditionally associated with

Black tourmaline is traditionally associated with protection and with boundaries. The threshold and the front door in particular. This is the stone most often kept by an entrance or in a corner of a room in modern practice, and it is commonly set out at the start of a working rather than the end. 

How I'd keep it

Hardness helps here, but the piece in the shop is a natural mix of tourmaline, quartz and feldspar, and feldspar is the soft one of the three. Rinse in cool water, dry it straight away with a soft cloth, and keep it away from acids and household cleaners, which will cloud the feldspar long before they touch the tourmaline. It is stable in sunlight and won't fade. A piece like this wants a shelf or a windowsill where the prism faces catch light and the striations show.

What to watch for

Black tourmaline is one of the most faked stones on the market. Dyed agate sold as black tourmaline is everywhere, and it gives itself away by being perfectly smooth with no striations. Heat-treated and irradiated material is also common, and so is "black tourmaline" that is really slag from a foundry. Real schorl has visible lengthwise grain, a slightly glassy surface and often a duller-looking end where the crystal broke.

In the shop

The Black Tourmaline with Quartz and Feldspar at £35 is the piece I carry. It is a natural pegmatite specimen rather than a solid black crystal, so the photograph is worth a proper look. What you'll see is black tourmaline set through quartz and the cream-coloured feldspar it formed beside.