Gym Chalk – Magnesium Carbonate for CrossFit, Weightlifting, and Gymnastics 100% Pure Magnesium Carbonate gym chalk for weightlifting, muscle ups, pull ups, and other Crossfit® exercises. 8 – 2oz blocks come in the 1lb box. Our gym chalk is pure chalk and sticks to the hands and or bar/kettlebell extremely well. Inferior gym chalks are very powdery and fine and offer very little adherence.