Sphalarite (ZnS) or Zinc sulfide & sometimes called Black Jack.
Location = Mexico
Physical size of specimen = 1.5” long x 1.25” wide x .5” thick
This physical specimen actually appears to be very shinny and bluish-black, but the reflective light has created prismatic colors.

Sphalarite (ZnS) or Zinc sulfide, sometimes called Black Jack
Location = Mexico
This image is a fragment (1/4” long x 1/8” wide x 1/16” thick) removed from the above pictured specimen.
Image created with B&L SZ5 microscope @ 10x, coupled to a ccd camera and computer.
This digital image color is very close to the physical.
No sublimate formed even though prolonged heating was employed. Heat has changed the over all color of crystal to brownish. A little amount of minute of a greenish brown encrustation, which may be traces of copper, but the torch flame did not indicate any copper. The applied heat has also slightly rounded a few of the crystal edges.
The crystal rests in a carved out depression to help keep the mineral in confined area so the blast from propane torch would not dislodge it.
Additional blowpipe and simple chemical tests can be applied to confirm or deny if barite.
Note: when applying heat to any rock fragment I slowly warm it first, then gradually allow the higher heat to engage the fragment to avoid blowing up, bursting or spitting sending tiny fragments all over the tablet.