Crux Power Performance Sports Chalk for Rock Climbing, Gymnastics, Weight Lifting - 100% Pure Magnesium Carbonate, No Drying Agents, Medium Blend Loose Rock Climbing Chalk - 200g : Amazon.co.uk: Sports & Outdoors
Annapurna Magnesium Carbonate Chalk Blocks [8 Pack] • 100% Pure chalk keeps hands dry for a secure grip • Ideal for climbers, gymnasts and weightlifters : Amazon.co.uk: Sports & Outdoors
TIL climbing chalk absorbs water at the molecular level, going from magnesium carbonate MgCO₃ to what chemists call "hydrated", but still dry-to-the-touch, hydromagnesite Mg₅(CO₃)₄(OH)₂·4H₂O. : r/bouldering