This instrument will show you if a gemstone is singly or doubly refractive. For instance, Moissanite is strongly doubly refractive, diamond is singly refractive. This is, therefore, a very cheap (if a little fiddly) method of distinguishing diamond from Moissanite.

You cannot, however, use this instead of a diamond tester because Cubic Zirconia , glass and plastics give the same reading as diamond.

At a more advanced level: glass and plastics are singly refractive, crystalline gemstones are mostly doubly refractive, excluding the cubic system of crystals (which includes diamond and cubic zirconia). So knowing if a gemstone is singly or doubly refractive is diagnostic.

How to use:

At the top and bottom of the tube are two filters (Polaroid discs), revolve the top filter until everything in the tube appears dark and place the stone on the platform in the middle. Now turn the stone so that it revolves through 360°. If it appears to turn light-and-dark it is doubly refractive, if it remains dark it is singly refractive.