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.
In more detail
Diamond, moissanite and cubic zirconia all read over the limit with 1.81 liquid. So a standard refractometer cannot tell these three apart. Moissanite's double refraction does not show on it either.
How we measure it
Our Refractometer shows the OTL view: the scale stays dark down to a faint liquid line at 1.81. The liquid's own critical angle in a glass of 1.88 is 74.3°.
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.
- Contact liquid: 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.
- 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.
- Refractive index: Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.