Principle
Light changes direction and speed across materials. On a gem refractometer, the boundary between bright and dark fields is used to read RI against a calibrated scale.
How to take a reading
Clean the contact surfaces, apply a tiny amount of suitable contact liquid, position a polished facet and read the shadow edge under appropriate illumination. Rotate the stone when the geometry permits.
Interpretation and limits
Compare measured RI with the expected range for candidate minerals. Not every stone can be read directly: rough or curved surfaces, surface condition, low optical contact and the instrument range can restrict results.
Illustrated guide
Optical character and birefringence
Six editable SVG reference diagrams, including uniaxial and biaxial responses.
Frequently asked questions
What does a refractometer measure?
It measures refractive index, a numerical expression of how light propagates within a gemstone.
Can every gemstone be measured?
No. The instrument has a limited RI range, and measurement requires suitable optical contact.