Gemstones and Refraction Liquids

Liquids with a high refractive index (RI) are used in gemology to evaluate the refractive index of a gem or to separate one from another in a bundle. Another use is to allow a clearer vision of any inclusions in a gem while it is immersed in the liquid.

These liquids warrant some discussion because they can be poisonous, dangerous, and flammable. They are also very expensive; however, alternatives do exist. These alternatives are less aggressive and less expensive dense oils. Among several oils that we tested, glycerol, or glycerin, has a RI of 1.47. While there are other oils that have desirable refractive indices, many of the oils we tested had a very strong scent, and it can be difficult to clean a jewel dipped in those liquids. Another problem is the fact that you can buy an allegedly pure oil, but when checked with a drop in the refractometer, it shows a RI so low that immediately we can conclude that the pure oil is a dilution. Any time you use a new liquid in the lab, always check it in the refractometer to confirm it is pure before immersing your gems in it.

Let’s look at a table with the chemical compositions and other data of some liquids commonly used in gemology.

carbon tetrachloride 1.460 CCl4  D3 (1) US$96/liter
toluene 1.500 C7H8 or C6H5CH3  D2 US$34/liter
monochlorbenzene 1.526 C6H5Cl  D5D4 US$55/liter
monobromobenzene, 1-Bromobenzene 1.560 C6H5Br  D5
O-Toluidine ,o-methylaniline,2-methylaniline 1.570 C7H9N  D2
bromoform 1.590 Br₃CH  D3 (1)D4 US$315/kg
1-Bromonaphthalene 1.660 C10H7Br  D5 US$34/100 ml
monoiodonaphtalene 1.705 C10H7I  D5
iodomethane 1.745 CH3I  D3 (1) US$75/250 g

As you can see, we have at least nine products with refractive indices ranging from 1.460 to 1.745. For a gemological lab, we advise having 200 ml of each liquid on hand. But why would we recommend that you have multiple liquids?

The main reason we recommend having multiple liquids in your lab is because you may need liquids with different refractive indices to see the inclusions and determine the RI of various gemstones. If you do not want to purchase these costly and dangerous liquids, there is another option. Take a look at the details provided for 1-Bromonaphthalene. Note that it has a good balance between price and RI. In addition, this product is slightly soluble in water, but is miscible with alcohol, ether, benzene, and chloroform. The color ranges from slightly yellow to deep brownish-yellow. Also, you can dilute the 1-Bromonaphthalene in alcohol, using the purest alcohol you can find, to create a set of refraction liquids that range from 1.400 to 1.660. To achieve this, dilute slowly, taking a sample drop of the liquid after each dilution to test in your refractometer. Each time you have a solution with a RI you want to keep, save a batch and label it before moving on to your next dilution.

Never forget these important rules:

1. Safety first. Avoid skin contact, use goggles; and a face mask. Keep fresh air in circulation, even using a small fan if needed. Keep children and animals away. In fact, if you have children, you probably should not use these liquids at all.

2. For each RI liquid, use one pipette with a paper tag indicating the respective RI liquid. Do not mix the tools between liquids or you will ruin your calibration work.

Other liquids, such as monoiodonaphtalene and iodomethane, are expensive and there is no way to replace them. The iodomethane is quite useful, but remember that it will cost you around US $75 for only 250 grams. The issue is that this product is a very dense, and colorless, liquid. One cubic centimeter (cc) of it weighs 2.28 grams. For 250 grams you only get approximately 110 cc of liquid. That is not much, as we found that 200 cc (or ml) is a good volume to have in the lab. If you could not afford to pay US$150, try to use the least possible amount of this liquid. Also, for your safety, always remember: THIS LIQUID IS POISON.

To inspect inclusions in a gemstone, you do not need to completely immerse the piece in the refraction liquid. You can use the technique called partial immersion. This technique consists of placing only a drop of liquid over the gem and then observing it. With this technique, you will be able to spare your liquids, minimize the risk of skin contact, and you will not be exposed to the vapors. See Figure 1.

Figure 1
Figure 1: This figure shows the partial immersion technique.

In case of total immersion, you can use a mount with a 45 degree mirror, as we show in Figure 2.

Figure 2
Figure 2: This illustration shows a setup for total immersion incorporating a mount with a 45 degree mirror.

We prefer a adapted plastic black box as shown in Figure 3.

Figure 3
Figure 3: This photo shows an adapted black box that can be used for total immersion. This is the device we prefer.

Finally, to jog your memory, in Figure 4 we show you what the gemstones will look like when they are immersed in those liquids.

Figure 4
Figure 4: This diagram shows what you can expect to see when immersing gemstones in various refraction liquids.