by Ritika Changrani
A jeweller’s loupe is a tool that helps identify the internal flaws, cracks and blemishes on the surface and any other cover ups on the gemstone.
A Loupe is just a magnifying glass. It helps to see the magnified image of gemstone and serves as the identification and grading tool. A Loupe is made of a powerful convex lens. Through a jeweller’s loupe, one can easily see the internal flaws, cracks and blemishes on the surface and any other cover ups on the gemstone. Jeweller’s loupe is the most important test equipment for testing gemstones.
How to Choose a Jeweller’s Loupe?
There are two important factors to be considered while choosing the right jeweller’s loupe. They are the size and magnification of the lens and the optical quality. Loupes are available in different sizes and powers. The standard size used for testing gemstones is of 18mm length with 10X, which magnifies the gemstone to ten times its size. The jeweller’s loupe is available in 20X and 30X, but they are not much used.
The jeweller’s loupe has to be tested for achromatic and spherical aberrations. Chromatic aberration refers to colour fringes of the image due to dispersion caused inside the glass lens. The loupes which are corrected from chromatic aberration are known as achromatic. Spherical aberration refers to the distortion caused due to the differences in refraction on the outer edges of the lens and the rays traveling through the center. The loupes which are corrected for spherical aberration are termed as ‘aplanatic’.
While choosing a jeweller’s loupe it is necessary to check that they are achromatic and aplanatic to get accurate results. In a better quality jeweller’s loupe, three lenses (known as triplet loupe) are used so that the field of vision is in focus to the edges and no false colour is imparted to the eye. The use of three lenses eliminates the pin cushion distortions and chromatic and spherical aberrations.
While selecting a jeweller’s loupe you can easily identify if they are achromatic and aplanatic. Look at the white light through the loupe. If the white light remains white without changing the colour, the lens is corrected for chromatic aberration. Similarly spherical aberration can be tested by looking at the mm squared drawing paper. If they remain squares when looking through the loupe, the loupe is aplanatic.
It is also better to choose a loupe with black framing around the lens, since this avoids reflections which may alter the color of the object under view. Do not opt for golden or other bright coloured frames since they interfere with the colours.
How Does a Jeweller’s Loupe Work?
Jeweller’s Loupe magnifies the image of the stone under test as per its magnification power. Suppose, we are using a 10x loupe, the image is magnified to ten times its original size. This helps us clearly see the flaws, cracks and other distortions in the stone. A triplet loupe consists of one ‘plano-convex lens’, one bi-concave lens and one bi-convex lens. The ‘plano-convex lens‘ is used to eliminate spherical aberration and the other two are used to eliminate chromatic aberration. Depending on the observations, the nature of the gemstone can be detected. Jeweller’s loupe helps to detect diamonds, sapphire, alexandrite, ruby, pearls and many other gems.
How to Use a Jeweller’s Loupe
- Remove your spectacles if you are wearing one. Unfold the jeweller’s loupe.
- Hold the jeweller’s loupe between the thumb and the index finger. Keep the jeweller’s loupe as close to the eye as possible without touching the eyeball. The loupe should be so close that your eyelashes almost brush through it. Once the loupe is close to your eye, see that it is kept stationary by letting the back of the thumb rest against the side of the nose. Keep the remaining three fingers in parallel or just below the index finger. Keep both your eyes open to avoid eye straining.
- Make sure there’s enough light in the place where you are observing the gemstone. Fluorescent and other light bulbs lack some colours which are necessary. Hence sunlight is the best light for the testing gemstone. Diamond lamps can also be used.
- Place a clean white soft cloth on the surface you are using. This helps to avoid scratches on the gemstone and also avoids rolling down in case it slips down. Next, place the stone at a distance of an inch or a little more on the other side of the gem loupe. Peering through the gem loupe, move the stone to and fro until it comes in focus. Keep moving the stone front and back and turn the stone in different directions until the area of the stone is seen with sharp focus.
- Note the external and internal features of gemstones. Look if there is unevenness in colour. If there is unevenness in colour, there is a possibility that the gemstone is dyed or painted to imitate a high quality gem. Many deceptions which cannot appear to the naked eye can be observed through the jeweller’s loupe. If you already have some real gems of similar types, you can observe both and compare them.
- Some of the things you should observe while testing the gemstone through the loupe are :
- Symmetry: Check if the gemstone has a balanced symmetry
- Number of facets: Observe whether the gemstone has the right number of facets required for the particular cut
- Sharpness of the facet edges: Hard stones have sharp edges while synthetic stones have soft edges
- Inclusions, flaws and bubbles in gemstones: Inclusions, if present, don’t necessarily indicate that the gemstone is bad. Instead, the gemstone can be a natural gem. Flaws in gemstones are not an indication that the gemstone is of low value. Bubbles in gemstone indicate that the gemstone is synthetic.
- Scratches and cracks in gemstone: Soft stones tend to scratch easily while hard stones crack.