Graphite
Graphite is carbon, hardness 1 to 2 and density 2.09 to 2.23. Molybdenite looks identical and is more than twice as dense, which is the only reliable test.
Graphite is native carbon, hardness 1 to 2, with perfect {0001} cleavage and a measured density of 2.09 to 2.23. Molybdenite shares its habit, lustre, greasy feel and softness but sits at 4.62 to 4.73 — more than twice as dense. Density is the only separation a collector can make reliably at home.
In short
- Graphite and diamond are the same element and sit at opposite ends of the hardness scale. Graphite is 1 to 2, diamond is 10. The Handbook lists graphite as polymorphous with chaoite, diamond and lonsdaleite — four species, one chemical formula, C.
- Molybdenite is the confusion that matters, and density settles it. Graphite 2.09 to 2.23, molybdenite 4.62 to 4.73. That is a factor of 2.15, which is far outside any measurement error a kitchen method introduces.
- Graphite is the least dense species with a metallic lustre this site covers, at 2.09. Only chrysocolla at 1.93 and sylvite at 1.993 are lighter, and neither is metallic.
- The streak separates them too, if you have paper rather than a plate. Graphite's streak is black to steel-grey and shining; molybdenite's is greenish to bluish grey. Graphite marks paper readily — the name is from the Greek for to write.
- Crystals reach 20 cm as hexagonal plates with triangular striations on the base, but almost all graphite is massive, foliated, scaly or earthy. The Handbook notes only a few of the numerous localities afford well-crystallised examples.
| Species | Formula | Hardness | Density | Streak | What settles it |
|---|---|---|---|---|---|
| Graphite | C | 1 to 2 | 2.09 to 2.23 | Black to steel-grey, shining | — |
| Molybdenite | MoS2 | 1 to 1.5 | 4.62 to 4.73 | Greenish to bluish grey | 2.15 times the density |
| Talc | Mg3Si4O10(OH)2 | 1 | 2.58 to 2.83 | White | Not metallic; white streak |
| Galena | PbS | 2.5 | 7.58 | Lead-grey | Cubic cleavage, and it is heavy in the hand |
| Stibnite | Sb2S3 | 2 | 4.63 | Lead-grey | Bladed prisms, not plates |
| Hematite (micaceous) | Fe2O3 | 5 to 6 | 5.26 | Red-brown | The red streak, and it is far harder |
The density test, done properly, on a specimen this soft
Graphite at 2.09 to 2.23 against molybdenite at 4.62 to 4.73 is not a marginal call. It is a factor of 2.15, and it is the reason the molybdenite page treats density as the whole determination.
The practical difficulty is not the arithmetic, it is the sample. Hydrostatic weighing needs a clean single-species fragment, and graphite and molybdenite both occur as scales and foliated masses in a matrix that is usually something else. A flake picked off a schist carries schist with it, and the measurement then returns the density of the mixture.
The method, the error propagation and the minimum specimen mass for a given scale resolution are set out in full on how to measure specific gravity at home. For a specific gravity near 2.2 the rule there gives a minimum mass of about 8.5 g on a 0.01 g scale — small enough that a good flake will do, if it is clean.
One element, four species, and the whole hardness scale
The Handbook of Mineralogy records graphite as polymorphous with chaoite, diamond and lonsdaleite. All four are carbon. Graphite is hexagonal or rhombohedral with perfect basal cleavage and a hardness of 1 to 2; diamond is cubic, with a hardness of 10.
Carbon therefore spans the entire Mohs scale on its own. No other element on this site's reference set does anything comparable — the structural difference between sheets of weakly bonded hexagonal rings and a fully cross-linked tetrahedral framework is worth the whole range of the scale.
Graphite also has two of its own stacking variants: a hexagonal 2H form in space group P63/mmc and a rhombohedral 3R form in R3m. Both are graphite. That is the ordinary situation for a layered species rather than an exotic one, and the same polytypism note appears on the muscovite page.
Borrowdale, and why English graphite matters out of proportion to its size
The Handbook's distribution list is worldwide — Ticonderoga and Franklin in the United States, Buckingham and Grenville in Quebec, Shunga in Karelia, Ratnapura in Sri Lanka, Passau in Bavaria, Pargas in Finland, Santa Maria in Sonora — and then, briefly: in England, at Barrowdale, near Keswick, Cumbria.
That is Borrowdale, and the deposit is the reason the pencil exists in the form it does. It produced graphite pure enough to be sawn into sticks rather than ground and bound, which no other known deposit did at the time. It is a small occurrence by industrial standards and a disproportionately important one historically.
For a collector the Cumbrian connection puts graphite in the same county as the Caldbeck Fells, although the two have nothing geological in common: Borrowdale graphite sits in hydrothermal pipes in the Borrowdale Volcanic Group, while the Caldbeck suite is a supergene assemblage in mineralised veins. Same county, different planet.
How to keep graphite, and what it does on a shelf
Graphite is chemically about as inert as a collection mineral gets. It does not oxidise at room temperature, it is not hygroscopic, it is not light-sensitive and it has no water of crystallisation to lose. Nothing in this site's reference set records a stability problem for it.
The damage graphite takes is mechanical and it is entirely down to the perfect {0001} cleavage plus a tenacity the Handbook describes as flexible but not elastic; sectile. It marks everything it touches, it takes fingerprints permanently, and a plate bent in handling stays bent. Handle it by the matrix, keep it in its own box, and do not let it share a drawer with anything pale.
The general standards for boxes, padding and drawer layout are on handling and storage. Species that genuinely need a controlled environment are a different problem, dealt with under storing soluble mineral specimens. Nothing here is offered for sale; the only commercial route on this site is the wanted list, and the rest of this section is indexed under mineral species.