GAS PHANTOM QUARTZ CRYSTAL
The faces have a sort of gassous-tree-like looking spray going like spreading fingers from bottom to top (base of triangle to tip).  My friend, Jay, who is a crystal mine owner, said that they are supposed to have every other face with the marking, one with the spray, one clear, one spray, one clear, etc...   They tend to have heavy etching (Sigils) on the faces. 

As for the gas phantoms, to describe them to you a little better, imagine on the face of the crystal... smoke.  starting from the flat (bottom) edge of the triangle, where you would get a small mouthful of smoke, purse your lips and blow very gently a thin stream of smoke.  The smoke would fan out like a tree in tendrils, whispy, in the best example.

Sort of a white-ish, faint blue-grey smoke (light see-throughy smoke), and the face surface of the crystal would be etched with triangular dents and ridges most of the time (Sigils), but sometimes smooth.  The smaller triangular faces would be clear (see-through) but the edges outlined with this thin rim of smoke.  All of this smoke is just under the surface, like about 1/16 of an inch.  The sides of the crystal would have just the hint of the "smoke", not whisps, solid, but maybe not so much of the look of it that is significant, they have a silky (almost lightly oily was a recent description of another friend of mine) feel to them.

This would be  a description of the very best example, of course.

As the crystals come from different layers in the strata, they have slightly different characteristics.