Agate

Agate is banded chalcedony, and chalcedony is quartz in crystals too small to resolve — cryptocrystalline, in fibrous aggregates rather than the visible prisms of ordinary quartz. The banding forms as silica-bearing solutions deposit in rhythm inside a cavity, usually a gas bubble in a lava, filling it from the walls inwards.

The fine fibrous structure has two consequences worth knowing. It makes agate slightly less dense and slightly softer than crystalline quartz, because of the porosity between the fibres. And that same porosity is why agate takes dye so readily, which is why the intensely coloured blue, green and magenta slices sold everywhere are dyed and why nobody in the trade regards that as controversial.

Moss agate and dendritic agate are not banded at all and are named by convention; strictly they are chalcedony with inclusions rather than agates.

Concentric or parallel banding in a translucent, waxy-lustred material with quartz hardness and no cleavage. Distinguished from jasper by translucency — jasper is opaque — and the two grade into one another in a single specimen often enough that the boundary is a judgement.

Ornamental stone, cabochons, beads, bookends and the polished slices that are its commonest form. Historically as cameo and intaglio material, where the bands are carved through to give a figure in one colour on a ground of another — the Roman and Renaissance work in sardonyx is the high point of that.

Industrially as agate mortars and pestles, and as knife-edge bearings in precision balances, both of which use its hardness and its lack of cleavage.

From the Achates, a river in Sicily, by way of Greek and Latin — Theophrastus records the derivation in the third century BC, which makes it one of the older stone names still in use.