Within the stone industry both of the terms are used without consistency. In science the word fissure is used to describe a major crack, break or fracture in the rock. In contrast, when used in the stone world, the term refers to a very slight separation, visible along intercrystalline boundaries. Mostly the differential is recognized by how it occurred. For instance the accepted standard is that a fissure is naturally occurring in nature, where as a crack is created by something in response to a stressor like handling, transport, fabrication, or installation.  The difference can be seen most easily by looking at a low angle.  A crack has two diffeent planes of reflection; fissure is stable, and part of how the stone was formed.