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3rd reading  135 degree rotation 4th reading

When taking refractometer readings, one usually starts with the largest facet (which is usually the table facet). Place your stone in the starting position, then close the lid of the refractometer. Make sure the light source is on.

Position your eye in front of the ocular in a way so that it is at a straight angle with the refractometer scale. You will now most likely see a dark region at the top of the scale and a lighter region in the lower part. If you have chosen a monochromatic sodium light source, there will be a sharp line between the lighter and darker areas. That line is named the "shadow edge". (You may also observe 2 less sharp "shadow edges".)

Place the polarization filter on the ocular and, while looking at the scale, turn the polarizer 90 degrees left and right. You will observe either of two possibilities:

1. only one shadow edge is seen  the stone is either isotropic or

 the incident light reaches the stone at an angle parallel to the optic axis and you should turn the stone 90 degrees

2. you see the shadow edge move between two values on the scale  the stone is uniaxial or

 the stone is biaxial

 In the first case, where only one shadow edge is seen, the reading for the shadow edge will remain constant during a 135 degree rotation of the stone. For every rotation reading, take two measurements: one with the polarizing filter in North-South position and one with the polarizing filter in East-West position.

Introduction to Gemology

The readings in the images below indicate a single refractive (isotropic) stone with RI = 1.527, which is most likely glass. (If one finds a single refractive transparent faceted stone with an RI between 1.50 and 1.70, it is most likely glass). Taking four sets of readings (with the polarizer in both positions) on a single refractive stone looks like overkill, which it is; take them anyway.

First reading Second reading Third reading Fourth reading

1.527 1.527 1.527 1.527 1.527 1.527 1.527 1.527

 In the second case, where the shadow edge moves between two values on the scale, write down both values you see, in table form below each other.

Below are 4 sets of readings of a double refractive stone with a uniaxial optic character (where one reading value remains constant). For every set of readings, you rotate the stone 45 degrees with your fingers without applying pressure while leaving the stone in contact with the hemicylinder.

First reading Second reading Third reading Fourth reading

Introduction to Gemology

While taking your refractometer readings, write down the values you read on the scale. For every set of readings, the polarization filter is turned 90 degrees. In addition to this you can also take a fifth reading (180 degree rotation).

In the example above, the lower readings (1.544) stay constant while the higher readings vary. In other gemstones, the higher value may remain constant while the lower value changes.

Note: The lower reading is the reading of lower value, not lower on the scale.

The RI of this stone is 1.544 - 1.553 (smallest lower reading and largest higher reading). This indicates quartz.

To calculate the birefringence of the gemstone being tested, you take the maximum difference between the largest higher reading and the smallest lower reading. In this example, that is 1.553 - 1.544 = 0.009 .

Some gemstones have a lower reading that falls within the range of the refractometer (and the liquid), while the higher reading falls outside the range. Those gemstones will give you just one reading on the refractometer and should not be confused with isotropic gemstones.

 Gemstones may also have two variable lower and higher readings, but the procedure remains the same. You write down the lower and higher readings in a table and calculate the

birefringence.

First reading Second reading Third reading Fourth reading

1.613 1.619 1.611 α 1.616 1.614 1.619 1.611 α 1.620 γ

1st 2nd 3rd 4th

lower readings ω 1.544 1.544 1.544 1.544 higher readings ε 1.553 1.552 1.549 1.552

Introduction to Gemology

These readings give an biaxial reading with RI = 1.611-1.620 and a

birefringence of 0.009, indicating topaz. You may have noticed some odd looking letters in the image footers, like α, γ, ε, and ω (and β which will be seen

later on ). They are not typos but Greek letters whose meanings will become apparent in the discussion on optical sign. You will also learn why we added the "difference" in the biaxial table. Optical character

Optical character refers to how rays of light travel in gemstones (or most other materials). In uniaxial and biaxial materials, the incoming light will be polarized in two (uniaxial) or three (biaxial) vibrational directions which all travel at different speeds inside the gemstone. This is due to the molecular packing inside the stone. For a better understanding, we refer to the discussion ondouble refraction.

Gemstones are divided into three categories (characters) depending on the way a ray of light behaves as it passes through the stone:

1. isotropic 2. uniaxial 3. biaxial

 Isotropic stones are stones in which light travels in all directions at equal speed.

Among those stones are the ones that form in the cubic system as well as amorphous stones, like glass.

• On the refractometer you will see one constant reading.

 Uniaxial means that light travels differently in two directions.

One ray of light will vibrate in the horizontal plane, which we call the ordinary ray (ω). The other will vibrate in a vertical plane along the c-axis and is called the extra-ordinary ray (ε). This extra-ordinary ray is also the optic axis (the axis along which light behaves as if being isotropic).

Gemstones that are uniaxial by nature belong to the tetragonal, hexagonal and trigonal crystal systems.

• You will see one constant and one variable reading on the refractometer.

1st 2nd 3rd 4th difference lower readings 1.613 1.611 1.614 1.611 0.003 higher readings 1.619 1.616 1.619 1.620 0.004

Introduction to Gemology

 Biaxial gemstones split up incoming light into two rays as well, however the crystallographic directions are labeled as the α, γ and β rays. The two rays both act as extra-ordinary rays. Stones with a biaxial optic character have two optic axes.

The orthorhombic, monoclinic and triclinic crystal systems are biaxial. • This will be shown by two variable readings on the refractometer.

Spot readings (distant vision method)

This is the method used to estimate the RI of en-cabochon cut gemstones.

You place a very small drop of contact liquid on the hemicylinder and place the stone on the drop, on it's most convex side (as in the image on the bottom-right). Remove the polarization filter (if not already done) and close the lid.

Move your head back about 30 cm from the ocular and look straight to the scale. On the scale, you'll see a reflection of the contact liquid droplet. When you move your head slightly in a "yes-movement", you'll observe the droplet move over the scale. Try to fixate the point where half of the droplet is dark and the other half is bright.

The image at the top right shows three stages while moving your head. The top droplet is too light and the bottom one is too dark. The one in the center shows a good half dark/half bright droplet.

Introduction to Gemology

Now move your head toward the ocular and estimate the Refractive Index. Unlike with faceted gemstones, we estimate to a 0.01 precision when using this method. The image on the left shows the reflection of the liquid which is half bright/half dark at 1.54. This gemstone may be Amber.

Alas, one cannot determine birefringence using this method, unless the birefringence is quite large (as with the carbonates). The "birefringence blink" or "carbonate blink" technique makes use of a larger drop of contact liquid and a polarizing plate. As the plate is rotated, the spot will be seen to blink. A crude estimation of birefringence can be made by this technique.

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