What is climbing chalk actually made of?

Climbing chalk is magnesium carbonate, chemical formula MgCO3. It's not the same substance as blackboard chalk, which is usually calcium carbonate or gypsum. The two get lumped together because of the name, but they behave differently on skin.

Why magnesium carbonate specifically?

The key property is that it's relatively insoluble in water. Since sweat is mostly water, a substance that doesn't dissolve in it can sit on your skin and keep doing its job instead of turning into a slippery paste. Magnesium carbonate binds moisture on the surface of your hands rather than mixing into it, which is what keeps your grip dry instead of just temporarily masking the sweat.

At a practical level, the powder also fills in the fine ridges of your fingerprints, which increases the actual surface contact between your skin and whatever you're gripping. More contact means more friction.

Is it the same as what gymnasts use?

Yes, gymnastics chalk and climbing chalk are both magnesium carbonate, used for the same basic reason: keeping hands dry enough to maintain grip and reduce slipping.

Is magnesium carbonate safe to have on your skin?

For short-term skin contact, yes. It's a low-hazard substance and widely used exactly this way across multiple sports. It's not something you'd want to inhale in large quantities over time, which is part of why gym ventilation and dust management matter, but topical use is considered safe.

Does "pure" magnesium carbonate mean anything?

It refers to how much of the product is actually magnesium carbonate versus fillers or additives. POUFF is 100% pure magnesium carbonate, blended 80% chunky to 20% fine for texture, with nothing else mixed in.

The takeaway

Climbing chalk works because magnesium carbonate resists dissolving in sweat and binds moisture at the surface of your skin, which is a different mechanism than simply drying your hands off. That's the whole reason it's the standard across climbing, bouldering, and gymnastics alike.