The following article appears courtesy of GIA Gems & Gemology eBrief
Intellectual Property: Gemological Institute of America
Photo by: Nicholas Del Re
Author: GIA Gem Identification Course
The immersion technique involves submerging a sample in liquid. If the stone’s RI is close to the liquid’s RI, immersion makes the interior more visible by reducing the effects of refraction and surface reflection. This enables you to see a gem’s inclusions or color distribution more easily. Looking for surface-conformal color zoning, or color concentrations along facet junctions, will help you assess whether a ruby or sapphire has been diffusion treated. Immersion can also make it much easier to see crystal growth structures, which might help you separate natural from synthetic corundum. Features like curved growth striae in flame-fusion synthetics, or separation planes in assembled stones, are often far easier to see when the stone is immersed.
To perform this test, you need an immersion cell such as a small beaker. Fill it with methylene iodide, glycerin, mineral oil, or water. Methylene iodide works best, but it’s toxic and requires care in use. Water, mineral oil, and glycerin are safe substitutes that usually work well.
Place the immersion cell over a diffused light source, such as a microscope’s overhead diffused fluorescent light turned face-up, or place a diffuser, such as a piece of translucent plastic, over the microscope’s well light. Be extra cautious if you are using methylene iodide because the heat from the light can add to its toxic effects.
Gently place the stone table-down in the immersion cell and make sure the liquid fully covers it. Then examine the stone with magnification. You should find the interior details of the stone much easier to see and differentiate.
– GIA Gem Identification Course