Minerals and ores
How to tell the ores that matter with nothing but a streak plate, a knife and your hand: colour lies, streak and weight do not.
Home: Metals, Level 2 · Forge, level 1 of 4 in its track. Serves 5 steps across 2 subjects.
Every metal starts as a rock someone recognised. Colour is the least reliable clue, because the same mineral comes in many colours and tarnishes. The tests below, which need only a piece of unglazed pottery, a steel knife, a magnet and a drop of acid, tell the main ores apart. Each ore links to its element on the periodic table, which says how it is smelted and how it harms.
Five tests
- Streak: rub the mineral on the back of an unglazed tile or pot. The colour of the powder is constant even when the lump's colour is not. Hematite always streaks red-brown, whatever it looks like.
- Hardness: a fingernail scratches up to about 2.5 on the scale below, a copper coin about 3.5, and a steel knife or glass about 5.5. Pyrite scratches glass; gold does not.
- Weight in the hand: most rock weighs about 2.7 times its volume of water. Metal ores are far heavier: galena 7.5, cassiterite about 7, gold 15–19. A lump that feels too heavy for its size is worth a second look.
- Magnetism: magnetite pulls a magnet; hematite barely does.
- Acid: carbonates fizz in dilute acid. Limestone does, and so does malachite, the green copper ore.
The Mohs hardness scale
Each mineral scratches the ones above it in the table and is scratched by the ones below.
| Hardness | Mineral | Everyday comparison |
|---|---|---|
| 1 | Talc | Crumbles under a fingernail |
| 2 | Gypsum | Scratched by a fingernail |
| 3 | Calcite | Scratched by a copper coin |
| 4 | Fluorite | Scratched easily by a knife |
| 5 | Apatite | Scratched by a knife with effort |
| 6 | Orthoclase feldspar | Scratches glass |
| 7 | Quartz | Scratches steel |
| 8 | Topaz | Scratches quartz |
| 9 | Corundum | Ruby and sapphire; emery abrasive |
| 10 | Diamond | Scratches everything |
The main ores
The ones a rebuilding community is most likely to find and able to smelt.
| Ore | Metal | How to tell it | Streak |
|---|---|---|---|
| Hematite | Iron | Red-brown to steel-grey, often in rounded kidney shapes; heavy | Red-brown |
| Magnetite | Iron | Black, heavy, and pulls a magnet | Black |
| Bog iron (limonite) | Iron | Brown lumps in wet ground and bog bottoms | Yellow-brown |
| Malachite | Copper | Green, banded, fizzes in acid; green stains on rock point to copper | Pale green |
| Chalcopyrite | Copper | Brassy yellow, often with a rainbow tarnish; a knife scratches it | Greenish black |
| Cassiterite | Tin | Dark brown to black, very heavy; worn pebbles in stream gravel | White to pale brown |
| Galena | Lead | Silver-grey cubes that break into smaller cubes; very heavy | Lead-grey |
| Sphalerite | Zinc | Dark and resinous, sometimes yellow or red | Pale brown to yellow |
| Cinnabar | Mercury | Bright red; roasting it gives off poisonous mercury vapour | Scarlet |
| Native gold | Gold | Buttery yellow in any light, never tarnishes, soft enough to dent | Golden yellow |
| Pyrite, fool's gold | Iron and sulfur | Brassy, in cubes; hard enough to scratch glass; shatters when struck | Greenish black |
| Native sulfur | Sulfur | Bright yellow, light; melts and burns with a blue flame | White to pale yellow |
Build the gear
Where to go next
This page covers what you are most likely to meet. It is not a field guide, and on identification that difference can kill you. These are the works that go further — chosen because they can be held offline.
Still to come
Illustrations carry a technique further than prose, but some things only move well. Recorded here rather than left implicit.
- Photographs of magnetite, chalcopyrite and bog iron as found in the field, not as museum specimens