The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, gray to pink, commonly curved (saddle shape) crystals, although it is usually massive.
Many workers have tried to understand how massive dolomite forms, with little luck, leading to the term the “dolomite problem”.
Some dolomite forms in very particular evaporatite, high saline, environments, such as sabkhas in the persian gulf, where seawater (which is high in Mg) floods over a tidal area, percolates through to limestone, then gets evaporated by the sun, transforming the calcite in the limestone in to dolomite, something like this model:
Why am I boring you with this? One reason is that I spent an afternoon revising dolomitization and went so far as to read an article on the use of sulphate reducing bacteria in the formation of dolomite, and another is that dolomite has puzzled workers for more than two centuries!