Calcite forms a bewildering away of crystal shapes, most commonly bladed or in a distorted rhombus. Sometimes though it doesn’t form in one of its own shapes, but replaces crystals of another mineral, a process known as pseudomorphism. It occurs when the chemical conditions change, for example when heated mineralised waters pass through rock. The chemistry determines whether they will dissolve a mineral and leave another in its place or not. In the case of this beautiful white cube measuring 5.5x5.6x3.8cm, I’m not sure what it replaced.