Invertebrates have been producing calcified hard-parts (tests, skeletons, shells) for millions of years, and they have become very good at it. Biomineralisation is, in some ways, a straightforward process adopted by many phyla, but when examined in detail it is clear that different groups mineralise in different ways. Calcium carbonate is one of the preferred biominerals, and for good reasons: the ocean is full of its components, and organisms are already adept at handling them. Because of the shape and size of the calcium ion, calcium carbonate is a polymorphic mineral, so that biomineralisers have a range of substances at their disposal. They deploy minerals in many cunning ways, with design strategies ranging from “keep it simple” to “composition reflects function” to “aiming for wow”. How will these different strategies fare in a rapidly-changing ocean?