And as in some real rubies there are found slight hollows corresponding or analogous to the bubbles found in melted glass, it becomes a matter of great difficulty to distinguish the real from the imitation by such tests as hardness, specific gravity, dichroism, and the like, so that in such a case, short of risking the ruin of the stone, ordinary persons are unable to apply any convincing tests.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin
This stone is doubly refracting, exhibiting extremely strong dichroism, especially in the blue and the green varieties.
"The Chemistry, Properties and Tests of Precious Stones"
John Mastin