Agate, Jasper or Just Chalcedony?

The distinction people argue about is one of appearance, not chemistry, which is why nobody ever quite settles it.

They are all the same mineral

Chalcedony is quartz in microscopic fibrous crystals — too small to see, which is why it looks solid rather than crystalline. Agate and jasper are both chalcedony, and so are carnelian, chrysoprase, onyx and flint.

The names describe how a particular piece looks. They are not separate species, and no test distinguishes them because there is nothing chemically to distinguish.

The working distinction

Hold the stone against a bright light with a thin edge or a chip towards you. Glowing at the edge means agate; a sharp dark outline with no glow means jasper.

Where the argument comes from

Plenty of material sits in between, being translucent in parts and opaque in others. Dealers apply whichever name sells better, and localities have their own conventions that ignore the general rule entirely.

There is no authority to appeal to, so a stone that is half of each is legitimately called either.

Confirming it is chalcedony at all

Three checks, all covered in more detail in the streak test and testing hardness at home:

Hardness. It scratches glass easily and a steel knife will not mark it. That is quartz hardness, and it rules out most of the softer lookalikes.

Streak. White, regardless of the stone's own colour. A coloured streak means something else.

Fracture. Curved and glassy, like a broken bottle, rather than flat and stepped.

What it is not

Calcite. Much softer, scratched by a knife, and it fizzes in dilute acid where chalcedony does nothing. The full comparison is in calcite and quartz.

Glass or slag. Industrial slag is common on old sites and mimics agate reasonably well. It usually contains bubbles, which chalcedony does not, and it is often lighter than it looks.

Plastic or dyed stone. Dyed agate is sold everywhere and is not a fake as such, but the colour is uniform in a way natural banding rarely is, and it frequently concentrates along cracks.

Whether it is worth anything

Rarely much, because chalcedony is common worldwide. Value in this group comes from pattern rather than material — a distinctive locality, a well-defined scene in a picture jasper, or an unusual colour — and the general position in is my rock valuable applies without much modification.

The practical consequence is that a piece is worth cutting because it will look good, not because it will be worth money.

Cutting and polishing

Chalcedony takes a polish extremely well, which is why it dominates the tumbling and cabochon side of the hobby. It is hard enough to hold an edge, fine-grained enough to take a mirror finish, and common enough that mistakes cost nothing.

The one thing to check before cutting is whether the piece is sound. Agate nodules frequently have internal fractures that are invisible from outside and open up under a saw, and a stone that rings dull rather than sharp when tapped is usually cracked through.

Naming a piece honestly

If a stone is translucent and banded, calling it an agate is uncontroversial. If it is opaque, calling it a jasper is uncontroversial. Everything else is a judgement, and the honest description is the observation rather than the name: "translucent brown chalcedony with faint banding" tells another collector more than either label would.

That is generally true across mineral identification. The description survives disagreement about the name.