Black, and the seven blacks inside it

People who have never held them think black stones are interchangeable. People who wear them for a year can tell you which one is on their wrist with their eyes closed. The difference is not colour. It is weight, temperature, the way light leaves the surface, and what the bead does when it is knocked against a table.

Obsidian is the first black and the ground of the house. Volcanic glass, cooled too fast to form a crystal, so there is nothing inside it: no grain, no pattern, no flaw to catch the eye. Polished, it is a mirror. It is the softest of the seven, around 5 on the Mohs scale, and it is warm in the hand within a minute. Its tradition is the mirror's tradition: the old courts looked into it to see the room without its colour.

Onyx is the second and the one most people mean when they say black stone. Chalcedony, dense and even, near 7 in hardness, a hard bright polish that stays bright. It is cooler than obsidian, heavier, and it does not care what it is knocked against. It is the black of long negotiations, the classical stone of steadiness. If obsidian is a mirror, onyx is a wall.

Spinel is the third, and the smallest. We use it at four to six millimetres because at that size its hardness, 8, and its near-diamond lustre make a line of beads that reads as a single dark thread from across a room. It is the black of precision: small, exact, unbothered. It is the stone of the double wrap, worn where a cuff will show it.

Hematite is the fourth and is not black at all in the hand: it is steel, metallic, grey-silver at the edge of a bead and dark at the centre, and it is the heaviest of the seven by a distance. Iron. Tradition kept it for the days when the ground was moving. It is cold, it stays cold, and a strand of it on the wrist feels like ballast.

Lava stone is the fifth, and the opposite of hematite. Basalt, matte, full of open pores, so light it barely registers, and warm to the touch because it holds no cold. It takes a drop of oil and darkens. It is the black of what has already been survived.

Ebony is the sixth, the one that is not stone. Near-black wood, dense enough to sink in water, with a satin surface and a faint grain that only shows under the lamp. It is the warmest of the seven and the only one that changes with the wearer. The wood of long offices.

Smoky quartz is the seventh, and only black at its core. Hold it to the light and it is brown, then amber at the edges, transparent where the bead is thin. It is the keeper's stone: something with a centre that cannot be seen into, wrapped in something that can.

How to choose. If you want a piece that disappears under the cuff and is warm within a minute, obsidian. If you want one that will be knocked against tables for twenty years and not show it, onyx. If the wrist will be seen and the line should be fine, spinel. If you want weight, hematite; if you want none, lava. If you want it to change with you, ebony. If you want a black that is not quite black, smoky quartz.

The house strings them together on purpose, because a circle made of one black is flat, and a circle made of three is a conversation. Turn any of our darker pieces under a lamp and you will find at least two of the seven, and you will be able to tell which.