Milling minerals rich in magnesium and iron within CO2 gas has been proposed to capture carbon as metal-carbonates. Through a series of fortuitous events, we discovered that not only are rocks much better than trapping CO₂ than individual minerals, but that this process can occur on all common silicate minerals. Polymineralic rocks are crushed worldwide to produce construction aggregate. If crushing processes could be conducted within a stream of effluent CO₂ gas (such as produced from cement manufacture) our findings suggest that for every 100 Mt of hard rock aggregate sold, 0.4-0.5 MtCO₂ could be captured as a by-product.