Mueller Weightlifting Chalk is a solid form of magnesium carbonate. Available as a single 2oz block as well as a case of eight (price reduction for a case of 8).
Gym chalk is used by gymnasts, rock climbers and weightlifters to improve their grip. It does this by absorbing moisture from your palms and fingers therefore giving you a more secure, consistent grip.
A nice solid block that can crumble but is a harder chalk than our own branded.