Polariscope, handheld, with built-in light

Summary

A tube 90mm tall X 35m diameter, it detects double refraction.

Area to put the stone, 12mm diameter (OK for most stones and small rings).

Built-in light.

Detail

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.

Included: soft carry-case, built in light (takes two 2032 batteries, included) 

Is it easy to use?

A bit fiddly but reasonably easy. 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.

Apart from the above, this item is not supplied with instructions, it is intended for gemmologists or those who are studying gemmology.  

Limitations

The stone needs to be rotated on the platform. That's very easy with loose stones, but there isn't much room to fit stones mounted in jewellery, it has to fit onto the platform which is 12mm diameter.

Light must be able to pass through the stone, not a problem with transparent stones, you'll have to use a torch to shine light through semi-transparent stones, you cannot test opaque stones.

This is a standard gem tester used by gemmologists, it does not come with instructions, you are expected to know how to use it (a bit like a bicycle, it does not come with instructions telling you how to ride it).