This anomaly is due entirely to the alteration of crystalline structure, which, in the one case, is so changed by the diminution in heat as to cause the crystals to be so placed that they become Diathermal, or transparent to heat-rays; whilst, in the other instance, the crystals which so arrange themselves as to be Diathermal are, by a slightly increased temperature, somewhat displaced, and reflect, or otherwise oppose the direct passage of heat-rays, which, at the lower temperature, obtained free passage.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin
Thus certain stones become both opaque and Diathermal, and as the heat is caused to vary, so do they show the complete gamut between the two extremes of total opacity and complete transparency to heat-rays.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin