Contact liquid is the drop of high-index liquid between a stone and the glass of a refractometer. It fills the air gap so that light can pass from the glass into the stone.
In more detail
The most common liquid has an index of 1.81. That sets the highest index the instrument can read. It also makes a faint line at 1.81 on every reading.
How we measure it
Our Refractometer uses 1.81 liquid only.
Related terms
- Refractometer: A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
- Over the limit (OTL): Over the limit (OTL) is the refractometer reading of a stone whose refractive index is higher than the contact liquid's. The scale stays dark down to the liquid line, so there is no number to read.
- Shadow edge: The shadow edge is the line between the dark and the bright part of a refractometer scale. It falls at the refractive index of the stone.