Chalk vs Gypsum
Chalk and Gypsum are commonly confused, and 2 measured properties separate them — density most clearly.
Chalk vs Gypsum
These are different kinds of thing (rock, mineral). Fewer properties mean the same thing across both, so a like-for-like reading may not be available.
Chalk
A limestone made almost entirely of microscopic algal plates — soft, porous, white, and not what a blackboard stick is made of.
Gypsum
CaSO4·2H2O
A soft sulfate mineral that gives up its water when gently heated and takes it back on mixing — the basis of plaster.
| Property | Chalk | Gypsum |
|---|---|---|
| Cleavage Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | perfect in one direction |
| Crystal system Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | monoclinic |
| Density | 1.8–2.6 g/cm³ | 2.31–2.33 g/cm³ higher |
| Lustre Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | vitreous to pearly, silky in fibrous form |
| Mohs hardness Mohs hardness is an ordinal scale: values can be ranked, but differences and ratios between them are not meaningful. | 1–3 | 2 |
| Streak Only one of the two has a figure we can stand behind, so there is nothing to compare. | Not known | white |
Why these get confused
blackboard and sidewalk chalk have been gypsum since the nineteenth century, because it casts into sticks from a slurry and makes less dust — so the most familiar association with the word is not the rock at all
The confusion is a real and documented one, not a resemblance we have invented.