Grey aragonite is the same mineral as the blue, without the copper in it. Calcium carbonate, formed in layers, and where the blue stone takes its colour from copper, this one takes its colour from whatever else was in the water as it grew: iron oxides, manganese, traces of other minerals, and often nothing much at all.

The result is banding in cream, grey, white and honey, laid down in soft lines that follow the shape of the cavity it formed in. It is often called onyx marble, and I will come to that in a moment.

Aragonite of this kind is a cave mineral. It forms where calcium-rich water drips or flows, and each layer is a period of deposition, so the bands are a record of time in the same way tree rings are. Deposits of this quality come mainly out of Pakistan, Mexico, Turkey and Argentina, and the material is cut and polished into bowls, plates and display pieces because the banding reads best when it is polished flat across the layers. On the hardness scale it sits at three and a half to four, the same softness as the blue material.

Traditionally associated with

The stone reached the market as a decorative material rather than a talismanic one so hasn't gained a folk story behind it. 

In modern practice it is kept as a display piece, and it is grouped with the banded stones because of how the layers read across a polished face.

How I'd keep it

Treat it exactly as you would a shell, because that is what the mineral is. Keep it dry. Aragonite is slightly soluble in water, so washing it dulls the polish over time, and soaking is worse. Keep it away from acids and most household cleaners will etch it, and a dull patch on a polished face cannot be brought back. Because it is soft, a bowl wants to sit on its own rather than in a cupboard with harder things knocking against it. Dust it with a soft dry cloth or a brush. Keep it out of long spells of direct sun.

What to watch for

The name is the thing to watch. It is often called Onyx, or "Onyx marble". This is a trade term, not a mineral one. True onyx is a chalcedony, the same family as agate, banded black and white and sitting at six and a half to seven on the hardness scale. It is a different material entirely. What most bowls sold as onyx are is a banded calcite or aragonite, which is what this is, and the giveaway is hardness and the look of the banding. The soft, flowing layers of a sedimentary stone rather than the crisper parallel lines of a chalcedony. That said, the term is so embedded in the trade that the material is honestly sold under it, including by suppliers who know exactly what it is. If you are looking at a bowl elsewhere, the question worth asking is which one you are getting, because they need very different handling.

In the shop

The Grey Aragonite Cloud Bowl at £40 is the piece I carry, and it is a one-off, so the bowl in the photograph is the one that arrives.

Free UK shipping on orders over £50. Everything is sent from Luton, and collection is available if you're local.