If you see a welder marking out a piece of metal with what looks like chalk, or a tailor doing the same to cloth, they're likely to not be using standard blackboard chalk.
Plain chalk is calcite, one of the several forms calcium carbonate can take. Welders' and tailors' chalk, on the other hand, is "French chalk", a stick of solid talc, magnesium silicate. Ground up, talc is the base for talcum powder.
This was just another of the pieces of vaguely interlinked data that float around in my mind, until I discovered I could buy ten 125-by-12-by-5mm (about 5 by 0.5 by 0.2 inches) sticks of French chalk, plus a sliding metal holder with a pocket clip, for a grand total of 4.8 Euros including delivery to me here in Australia.
So I had to buy the darn things, of course, in order to hasten the day on which my flattened corpse will be discovered beneath a fallen pile of scientific, electrical, medical and engineering toys and curios.
The talc sticks are unexpectedly beautiful objects. They're very smooth, despite visible sawblade marks on the sides...
They're moderately fragile, of course, but quite dense, and much harder-wearing than calcite chalk. And I think they've been cut from solid mined blocks of natural talc, because they all have slight marks and veins and other inconsistencies, which become more apparent...
...if you shine a light through them.
(The backlight is my possibly-actually-antique flashlight.)
I think there are two reasons why you'd want to use talc rather than calcite for marking out. First, the mark can be more accurate, because although talc is the definitive soft material (scoring one on the Mohs hardness scale), it's actually quite a bit harder and sturdier than a stick of blackboard chalk, and thus won't wear much in the course of one line across metal or cloth. Calcite itself is much harder than talc, but calcite chalk is deliberately made porous and weak; French-chalk sticks are solid and waterproof. A stick of solid non-porous white calcite would rip the paint straight off your blackboard.
The second and probably more important reason to prefer French chalk for marking steel or cloth is that when you draw with a talc stick, you get a line of freshly-created talcum powder. I think this will stick better to a surface than a normal chalk mark, and resist being rubbed or shaken off as you join and cut and otherwise handle your metal or cloth.
(There could be chemical reasons for the choice too, for welders at least. Magnesium silicate is used in some high-temperature pottery glazes, and it's also used as a welding flux, for gas welding at least.)
The ability to precisely draw talcum powder onto a surface could be mechanically useful, too. When I was a kid I used talcum powder to lubricate Technic Lego contraptions, because it doesn't make much of a mess and doesn't attack plastic. Graphite powder, which you can similarly topically apply with a soft pencil or artists' graphite stick, is a better dry lubricant - but it turns everything black and conducts electricity, which may or may not be desirable.
Talc is also a high-temperature electrical insulator. You could easily carve and drill small custom insulators out of French-chalk sticks, or use them unmodified as formers for heating elements or what-have-you.
What I'm actually likely to do with my sticks of talc, of course, is just fiddle with them aimlessly and admire them for their surprising beauty.
I reckon I got value for money, just for that.