Whewellite is the dominant mineral in the plant kingdom, and one of the most important minerals within the surface veneer of our planet. It is intimately associated with the functioning of living systems and plays a significant role in the carbon cycle. The monohydrate form of calcium oxalate, whewellite, commonly occurs along with the less stable dihydrate form (weddellite). These minerals are not restricted to plants—humans may be painfully aware of whewellite as the principal mineral phase of urinary stones. Geologically whewellite is found in diagenetic settings, hydrothermal veins and in meteorites (carbonaceous chondrites), nevertheless the mineral is massively under-represented in the geological column given its high productivity in the biosphere. Reasons for this are explored, and it is argued that a better understanding of the relationship between whewellite and host organisms will have benefits for agriculture, medicine, and potentially strategies for CO2 sequestration.