The Refractive Index Fluid is Gemologist tool for identifying Refractive index.; product allows your refractometer to measure within the measuring range of 1.30 to 1.80 Refractive Index.; Approx. 3ml Refractive Index Fluid in every vial; Burgundy liquid can also be used to reduce or eliminate glare from a gem's surface, facilitating inspection of the gem's interior for inclusions and other defects.; Perfect for testing ALL of your stones! As the name implies, it is widely used with gem refractometers as a contact medium for the identification of gemstones.