Reading a Welsh Stream Bed

Walk almost any stream in the Welsh hills — the Mawddach, the Rheidol, the Ystwyth, the upper Teifi — and you are walking through broken geology. The bedrock of Snowdonia and mid-Wales is largely Cambrian, Ordovician and Silurian mudstone and slate, cut through by countless mineral veins that once carried lead, zinc and copper. Erosion does the rest. What you are looking for is not a scientific collection but a handful of recognisable characters: quartz, pyrite and slate fragments. Learn those three and the rest of the stream bed starts to make sense.

One habit will improve your eye more than anything else: look at every stone both wet and dry. Water fills the surface pores and sharpens lustre, so a quartz pebble underwater gleams like glass while a dull grey lump may turn out to be brass-yellow once you lift it and hold it against the light. Dry it on your sleeve and you see its true colour, coatings and weathering.

Quartz: The Commonest Find

Quartz is the mineral you will meet most often, and it is the one beginners most often mistake for something more exciting. In a Welsh stream it rarely appears as a neatly pointed crystal. Instead it arrives as white to milky pebbles, as grey-blue veins running through darker rock, or as chunky lumps stained orange-brown by iron from the surrounding soil.

• Lustre: vitreous — glassy on a fresh break, greasy-looking when worn smooth by tumbling. Never metallic, never earthy.
• Hardness: 7 on the Mohs scale. It scratches window glass and the blade of a penknife, and nothing in your pocket will mark it.
• Fracture: conchoidal, meaning a broken face curves like the base of a bottle, with a shell-like sheen.
• Stream wear: rounded, frosted pebbles with a slightly chalky surface and, sometimes, fine crescent chatter marks where one stone has struck another.

A streak test helps confirm it: quartz leaves no coloured streak, or only a faint white one, on an unglazed tile.

Pyrite: Fool's Gold and How Not to Be Fooled

Pyrite is brass-yellow, metallic and undeniably pretty, and it turns up in slate quarries, mine spoil and stream gravel throughout the old lead districts. It forms cubes and clusters, often with fine striations across the crystal faces, and it commonly sits in dark nodules within slate.

The trap is real: pyrite is brittle and hard, while gold is soft and malleable. Scrape a knife point across pyrite and you get a greenish-black powder; the same knife cuts gold like butter. Pyrite also tarnishes, going brown, purple and iridescent as it weathers, whereas gold stays gold. Stream-worn pyrite often shows a dull brass colour with rusty patches where it has begun to break down — and that breakdown is not kind to the lining of your pocket.

Slate Fragments: The Country Rock

Most of the dark, flat material in a Welsh stream is slate or mudstone, the rock the hills themselves are made of. Good slate splits into thin, even sheets along a single direction of cleavage, and it feels smooth and faintly soapy between the fingers. Colours range from blue-grey through charcoal to a purple-brown with a soft sheen.

• Lustre: dull to slightly silky where cleavage surfaces catch the light.
• Hardness: soft. A knife point scratches slate easily, which immediately separates it from quartz.
• Stream wear: flattens into rounded discs and slabs, so slate pebbles tend to be much wider than they are thick.

Tap a flat piece against another stone and you hear a dull, ringing clack — quite different from the dead sound of a shale pebble, which is softer still and smells faintly of mud when you wet it.

Using Lustre and Hardness Together

Lustre tells you where to look; hardness tells you what you have found. Carry a steel penknife (about 5.5 on Mohs), a scrap of window glass and an unglazed tile or the rough underside of a mug. Quartz scratches glass and resists the knife. Pyrite scratches glass but crumbles under the blade. Slate is marked by the knife without any trouble at all. Haematite, another common stream mineral, looks grey-black and metallic but leaves a red-brown streak — a useful warning that you are not holding quartz.

It is worth remembering that a stream-worn stone has lost most of its diagnostic clues. Crystal faces are gone, colour is muted by iron staining, and soft minerals have already been washed away. Work with what remains: weight in the hand, the sound when tapped, the feel of the surface, and the way light sits on it.

Collecting Sensibly in the Field

A few practical points before you fill your pockets. Take only loose material from the stream bed and banks, never hammer bedrock or quarry faces, and never collect from a Site of Special Scientific Interest without permission. Old mine spoil can be unstable and may carry heavy metals, so wash your hands afterwards and keep stones away from your mouth. Leave the flat, disc-like slate pebbles in place where you can — they armour the channel and protect the bed from erosion — and pick a handful of quartz or a single decent pyrite cube instead.

Finally, note where your finds came from. A quartz vein in a stream almost always trails back to a source uphill, and a scatter of pyrite in the gravel may point towards an old level or adit. The pebbles under your boots are a map of the hills above, if you take the time to read them.