Because corundum is such an important gem species and the major problem in testing is to distinguish the natural from the synthetic, detailed suggestions for making this separation are reserved for Assignment #34.
In this later assignment, the student will become familiar with the various synthesizing processes, a knowledge of which is essential to an understanding of the characteristics of synthetics. Therefore, the information presented here is confined to suggestions for separating natural corundum from its most-often encountered imitators, both natural and manufactured, with the exception of its synthetic counterparts.
Despite the fact that garnet is singly refractive and ruby doubly refractive and the colors are rarely the same, those with little experience in gem testing seem to have more difficulty in separating these two stones than any others of natural origin. The reason for this is that garnet frequently shows strong anomalous double refraction and may therefore appear doubly refractive in the polariscope. When a dark-red stone with a refractive index of approximately 1.77 becomes alternately tight and dark in the polariscope's dark position, it should be checked further with the dichroscope. (An alternate means of checking for dichroism is to rotate the stone between the Polaroid plates in the polariscope light position.) A ruby will show strong pleochroism, which can be observed in the dichroscope or in the polariscope's light position. This may be checked further by rotating a Polaroid plate before the eyepiece of a refractometer: if the stone is doubly refractive, in the direction of maximum birefringence the two readings will be evident as a slight movement of the shadow line that indicates the R.I. as the plate is turned. Also, if the tester has developed an ability to distinguish doubling of back-facet junctions, even in stones of fairly low birefringence, he should be able to detect the moderate doubling in a faceleted ruby. Further, using long wavelength ultraviolet light in a darkened room will show fluorescence in ruby, but never in garnet. Finally, the absorption spectrum of ruby is entirety different from that of garnet when the stone is viewed through the spectroscope.
Since a garnet and glass doublet may have a color that is virtually indistinguishable from that of ruby and an R.I. on the table within the range of corundum, it is also frequently mistaken for ruby. Immersion in a high-index liquid is not as helpful in detecting an all-red doublet as
- Varieties of Corundum, Rubies, Sapphire
- Formation of Corundum
- Sources of Corundum, Rubies, Sapphire
- Polishing Rubies, Sapphire
- Physical & Optical Properties of Corundum
- Test and Identification of Corundum
