Moonstone is feldspar. That is the first thing to hold on to, because everything else follows from it. Where amethyst and rose quartz are varieties of quartz, moonstone is a completely different mineral family, and it behaves differently in the hand. Feldspar makes up most of the earth's crust, and it is rare for any of it to come out of the ground looking like this.

The glow is called adularescence, and it comes from the way the stone is built. Moonstone is not one uniform mineral. It is orthoclase feldspar intergrown with layers of albite, stacked in alternating sheets a fraction of a hair apart, and light entering the stone scatters between those layers and comes back out as a soft moving sheet of light across the surface. The colour of that sheen is usually blue or white, but the material from India can give a warmer cream sheen and Sri Lankan material is prized for the deepest blue. The effect moves as the stone is turned, and it is the reason moonstone is usually cut as a smooth cabochon rather than faceted. A flat polished dome is what releases it.

Now the naming, because it matters. Rainbow moonstone is not moonstone. It is white labradorite, from the plagioclase side of the feldspar family, and it shows the same play of colour as the dark labradorite on the shelf next to it, just over a white body instead of a grey one. It is a genuine, lovely stone and it has been called rainbow moonstone by the trade for so long that the name has stuck, but the two are different minerals and only one of them is moonstone. The black and dark grey versions sold as black moonstone are labradorite as well. On the shop, the Rainbow Moonstone Wand is the labradorite, and it is named as the trade names it.

Moonstone itself is mined in Sri Lanka, India, Madagascar, Myanmar, Brazil and Tanzania, and the old Swiss locality at Mount Adular is where the older name adularia comes from. On hardness it sits at six to six and a half, softer than quartz, and feldspar has planes inside it, along which it prefers to break. That is worth knowing before you handle it, because a sharp knock in the wrong place will snap it. It is a June birthstone.

Traditionally associated with

Moonstone carries one of the oldest sets of associations of any stone here, and it is unusually consistent. The Romans believed it was formed from solidified moonbeams, and Pliny wrote that it changed appearance with the phases of the moon. It was carried by travellers in the belief that it offered protection on a journey, hence the old name traveller's stone, and in India it has long been regarded as a sacred stone. It is traditionally associated with the moon, with tides, and with cycles rather than fixed states.

How I'd keep it

Treat it more gently than a quartz. Warm water with a soft cloth is fine, dry it straight away. 

Keep it out of acids and household cleaners. 

Store it separately from harder stones so it does not take scratches. 

A bracelet should come off before anything involving tools, gardening or a dishwasher.

What to watch for

Moonstone is one of the stones that can arrive with a fracture filled with resin to hold a crack together, and a filled crack will show as a slight change in how light passes through a line in the stone. Most moonstone on the market is untreated, and the stone is common enough that heavy faking is not worth anyone's time.

In the shop

There are two pieces. The Rainbow Moonstone Wand at £15 is the labradorite, a palm-sized wand with the black flecks of tourmaline running through it, and it is the one to look at if you want to see the flash move. The product page includes a video so you can watch the sheen travel across it. The Moonstone and Sterling Silver Triquetra Bracelet at £80 is set with the pale oval cabochon and the Triquetra detail.

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