Calcite Diamond - Mexico

 A brightly fluorescent calcite scalenohedron crystal enclosed in a non-fluorescent calcite matrix. The formation of this specimen causes the internal calcite to have a zoned diamond shape.

These types of complex specimens are formed by multiple generations of calcite growth and manganese oxide minerals, with the end result producing a brightly fluorescent central calcite scalenohedron, encased in layers of essentially non-fluorescent calcite and outlined in black manganese oxide.  

Color zoning can be created by changes in the nature of the fluid from which the crystal is growing.  The changes could be due to temperature, pressure, and/or chemical changes. In the case of fluorescent zoning, there can be a change in the activator (what causes fluorescence) that is incorporated in the growing crystal. Source: UV Waves, v50 n6, Glenn Waychunas