Fluor Spar (from which name the term is derived) shows it well, as does also Amber.
Absorption
bands in the spectrum are only seen in two minerals ; in both cases
their discovery was due to Professor Church. Zircon shows some black
bands when examined in the spectrum of white light, due to the presence
of traces of uranium. Almandine also shows some black bands, in this
case in the green portion of the spectrum.
The
effect of Rontgen rays on gems is now of much importance, especially in
the case of the Diamond, for this is very transparent to these rays,
while many of its would-be imitators, as glass, Quartz, white Topaz,
etc., are opaque, and these cast shadows. So also the red and blue
shades of Corundum (Ruby and Sapphire) are partly transparent, while
their imitators, Balas Ruby (Spinel of rose red colour) and blue
Tourmaline, are opaque. Exposure to Rontgen rays or to the emanations
of radium may cause some minerals to phosphoresce (e.g. Kunzite);
and Crookes has shown that some minerals phosphoresce strongly when
exposed in a high tension electric current in a very rarefied
atmosphere. Thus Ruby shows a strong red light, Sapphire a blue, and
Diamond a bright green light.
Subjoined is a table of the refractive index of the principal gems:—
