<P> An apatite crystal sits front and center on cherry - red rhodochroite rhombs, purple fluorite cubes, quartz and a dusting of brass - yellow pyrite cubes . </P> <Li> <P> Boules of silicon, like this one, are an important type of industrially - produced single crystal . </P> </Li> <P> Boules of silicon, like this one, are an important type of industrially - produced single crystal . </P> <Li> <P> A specimen consisting of a bornite - coated chalcopyrite crystal nestled in a bed of clear quartz crystals and lustrous pyrite crystals . The bornite - coated crystal is up to 1.5 cm across . </P> </Li>

What is a crystal and how does it form