There are circumstances when one can not use the gemological refractometer but still needs to know the approximate index of refraction of a gemstone.
It is no secret that when you insert a drop of water in a glass filled with water, the drop will will disappear. Although there are other reasons to why that happens, the focus now will be on the indices of refraction of the drop of water and the water in the glass.
As the drop has the same index of refraction as the water that is already in the glass, light will travel through it at the same apparent speed. This means that the drop of water will not bend the light more (or less) than the water it is now in (the water filled glass); the drop of water will disappear like magic.
If we were to drop a small glass bead (RI ~ 1.5) in the water (RI ~ 1.3) then there is a difference between the two indices of refraction and you will be able to see the glass bead in the water (although not as clearly as when it was in air (RI ~ 1). The glass bead will refract the light more than the surrounding water and this can be a very useful tool to determine the index of refraction of the glass or any other transparent material like a gemstone.
It should be clear by now that the closer the indices of refraction between the two media (liquid and gemstone), the more the stone will disappear when it is immersed in the liquid.
The famous gemologist B.W. Anderson devised a technique to make this visible with the aid of a series of comparative liquids.
When a cut gemstone is place table down inside a dish of water, the gem will act like a lens if the RI of the gem is larger than its environment (in this case water). Because of the difference in RI between the gemstone and the liquid (not just water) the gemstone will cast an inward shadow on the bottom of the glass. This shadow will be shown as a dark rim around the contour of the gemstone and the wider the rim, the larger the difference in indices of refraction between the stone and the liquid.
By placing a gemstone in a series of liquids with increasing (or decreasing) indices of refraction one can estimate the RI of the stone at hand. The smaller the rim, the closer the RI of the gemstone matches the RI of the liquid.
In order to observe this rim one only needs to insert a small mirror below the glass to redirect the image to the eye. You could of course lift the the glass and observe from below, but that would require a ceiling lamp.
As the lightsource needs to above the immersion cell (the glass beaker) it is advisable that the light is diffused before falling on the mirror. For this purpose one can use an immersion cell with a frosted bottom or create the diffusion by a thin sheet of white toilet paper (peeled).