The Spot Method

For normal use with the refractometer, the stone being tested should have a well-polished flat facet. In 1948, Lester Brown developed a technique which enables the gemologist to make refractive index readings on cabochons or stones with extremely small facets, which up until that time was not possible. The technique is known as the spot method or distant vision technique and works as follows:

- Remove the eyepiece and use white light.

- Put a very small drop of liquid onto the center of the hemisphere. Very gently place the stone onto the drop and then examine the scale.

- If the drop outline covers more than two or three scale divisions it is too large and the reading will not be accurate. The size of the spot should be reduced by picking up the stone, wiping the liquid off the stone, and putting it back down on the remaining amount of liquid still on the hemisphere.

- When the spot is no longer than three scale divisions, the head should be moved up and down the scale. The spot will go from dark to light as you move your head down the scale. The point should be found where the spot appears exactly half light and half dark. This is where the reading is made.

- Sometimes the spot changes so quickly from dark to light that the half position can not be seen. If it is dark at 1.58 and light at 1.60, the refractive index can be estimated at about 1.59.

The readings obtained using the spot method may be unclear and hazy. This may be due to the stone being poorly polished. In order to insure that the reading will be as sharp and clear as possible, it is good idea to examine the stone carefully to find the smoothest and best polished area. Then, that area should be put in contact with the hemisphere instead of a dull or scratched area. This will make a big difference in the accuracy of the initial reading. The first thing to do is to remove some liquid to sharpen the division. If the reading is still unclear then a rough estimate must be made, keeping in mind that it will only be accurate to plus or minus a few scale divisions.