What's the difference in a reflectometer and refractometer?

Question

I need to identify some minerals, and want to know which is most accurate & the simplest to understand. I would enjoy corresponding with any rockhounds out there.

Answer 1

Reflectometers do not in fact measure the refractive index of a gem or mineral but measure the ratio between the intensity of an incident light ray to that of the ray reflected back from the stone. Then using Fresnel's simplified equation multiplied by 100 calculate the percentage of perpendicular light reflected back from the surface of the gemstone (e.g. taking n=2.42 for diamond and n=1.55 for quartz you get figures of 17% for diamond and 4.6% for quartz). In reality there are many problems involved with this system, it is nearly impossible to light the stone with a perpendicular ray and measure the reflection at the same time so the ray is offset introducing inaccuracies. Other problems are that the stone must have a nearly perfect polish, be free from scratches and have no surface films (like finger marks) at all. There are also other problems regarding the wavelength of the light used and birefringence etc. All in all these instruments only give a reasonable reading when the conditions are perfect and even then it is only an approximation.
Refractometers on the other hand give a direct reading of the R.I. of a stone using the phenomenon of critical angle provided the R.I of the stone is lower than that of the dense glass of the refractometer and the contact liquid. In all practical terms the refractometer is the better instrument to use.

Answer 2

one reflects the other refracts