Graphite
C · Hexagonal · also called plumbago, black lead
Graphite is pure carbon arranged in sheets that slide over one another, which is why it feels greasy and writes on paper. It is the softest metallic-looking mineral you are likely to encounter, and the weight difference from galena or molybdenite settles most confusion.
Properties
- Mohs hardness
- 1–2
- Streak
- black, gray
- Lustre
- metallic, dull
- Colour
- Steel gray to black with a slightly greasy sheen.
- Cleavage
- Perfect basal cleavage; peels into flexible but non-elastic flakes.
- Fracture
- Flaky
- Specific gravity
- 2.1–2.3 (water = 1)
- Magnetism
- Not magnetic
- Acid test
- No reaction
- UV
- Not fluorescent
Tests that confirm graphite
- Marks paper — it is literally pencil lead.
- Greasy, slippery feel between the fingers.
- Very light, in sharp contrast to the metallic minerals it resembles.
What graphite gets confused with
Each of these is settled by one test. Run that test first.
Molybdenite is noticeably heavier and bluer, and leaves a greenish-gray mark on paper where graphite leaves black.
Galena is three times heavier and breaks into cubes.
Where it is found
Metamorphosed carbon-rich rocks — marble, schist, and gneiss.
Is it worth anything?
No specimen value in massive form. Well-formed crystals are uncommon and collectible.