Indicator Stones (Heavy Liquids)

In order to check the SG of the liquid, indicator stones are used. Indicator stones are small stones of a known SG that are kept in the bottles.

Heavy liquids and indicator stones
- 3.32: Methylene iodide: 100% = corundum (SG=4.00); jadeite (SG=3.34)
- 3.05: Methylene iodide: 68%; benzyl benzoate: 32% = tourmaline (SG=3.04-3.06)
- 2.85: Methylene iodide: 64%; benzyl benzoate: 36% = nephrite (SG=2.95)
- 2.67: Methylene iodide: 60%; benzyl benzoate: 40% = calcite (SG=2.71); syn.emerald flux (SG=2.66)
- 2.62: Methylene iodide: 59%; benzyl benzoate: 41% = crystalline quartz (SG=2.66); cryptocrystalline quartz (SG=2.60) Chalcedony
- 2.57: Methylene iodide: 58% benzyl benzoate: 42% = cryptocrystalline quartz (SG=2.60) Chalcedony; microcline feldspar (SG=2.56) Amazonite

These indicator stones are extremely valuable because they allow one to quickly estimate the density of the liquid. The number printed on the bottle is only a guideline regarding the liquid’s SG, and many factors such as evaporation or contamination can alter its SG. So it is of great importance that the tester checks the densities of the liquid daily. For instance, if you are using a 2.67 liquid and you find that calcite (2.71) floats very slowly, then the liquid must be slightly above 2.71. When a liquid’s SG does not exactly match the number of the bottle, it does not mean that the liquid needs to be adjusted. So long as you can find the SG of the liquid it can be used. This is the purpose of the indicator stones.

SG range of gemstones
When determining the specific gravity (SG) of a gem, one should always keep in mind that each gem has a plus or minus range of SG and not just a single figure. There are many factors which can affect the SG of a specimen, such as the clarity, inclusions, impurities, etc and a tester’s degree of error must be taken into account as well. So one can see that the final figure obtained is usually much less reliable than an R.I reading for instance. For this reason, it is usually not possible to separate one stone from another on the basis of SG alone unless their SG’s are apart from each other. You must always consider the plus or minus of a property. Although specific gravity is an important diagnostic property, in most cases, additional tests must be used to positively identity a particular stone.