Andrew's Birthday Rocks
from the team

Agate is banded silica that grows ring by ring inside a gas bubble in old lava. Cut one open and you get this: concentric rings around a hollow, and the hollow lined with tiny quartz crystals that catch the light.

The hot pink is a dye, the classic finish for decorative Brazilian agate, and it is a big part of the look. The colour sits deepest where the stone is porous, in the druse and the pits of the rind, and under a UV lamp the dyed bands glow a bright orange, which makes for a good party trick.

The sticker underneath says Made in Brazil, which is where it was sawn, polished and coloured. The agate itself almost certainly came out of the lava fields of Rio Grande do Sul in southern Brazil, the world's main source of this stuff.

Look at it from the side and you see the structure: closely packed quartz columns rising from a milky white base and ending in a crust of small, sharp points. That is a slice of geode lining, the crystal wall that grew inward from the rock.

The honey colour has a story. Quartz this colour, growing from white roots, is usually amethyst that was gently heated: heat turns amethyst's purple to yellow and brown, and the result goes by the name citrine. It may also be natural smoky quartz. Either way it is the same mineral, and the colour runs right through the crystals.

The six-sided points are classic quartz, each one a tiny pyramid with flat glassy faces, and the best of them sit right at the top of the crust where the light catches them.

The crystals are stubby quartz, the kind that lines a geode, and the shimmer is a vapour-deposited film: the cluster goes into a vacuum chamber, metal vapour (usually titanium) settles onto every surface, and the film is so thin that light interferes with itself in it, the same trick as oil on a puddle. Gold on the flat prism faces, violet and magenta on the tips, depending on the angle.

This finish goes by “titanium aura” or “flame aura.” Turn it over and the flat base is bare grey quartz, because the cluster lay face up in the chamber while the film settled.

Under a UV lamp it turns into a tiny mirror, which is exactly what a metal film should do.

Malachite is a copper mineral, a copper carbonate, and it grows where copper ore slowly weathers near the surface. It builds up in layers, which is why a polished piece shows rings, bull’s-eyes and rounded, knobbly sections: each ring is one more layer of green laid down over time.

This one was polished into a palm stone, and the pits and little open cavities came with it: they are where the layers grew around gaps, and one of them still has its crystal lining. A tiny pit near a cluster of knobs holds a dab of clear resin, a finishing touch the polishers use.

It came with a card naming Katanga Province in the Democratic Republic of the Congo. That is the Congolese side of the Copperbelt, and the mines around Kolwezi there supply much of the world’s malachite for polishing, so the card and the stone agree.