If there is one iconic instrument of the gemologist, it is the 10x magnifying glass. When you imagine a gemologist at work, the first image that usually comes to mind is that of a person admiring a gem with a magnifying glass.
The apparent simplicity of the magnifying glass is deceptive. In gemology, we use a very specific type of magnifying glass, called a triplet. The lens is actually made up of three lenses glued together. This is used to correct two types of distortions that are found with single lens magnifiers.
The first distortion is a geometric distortion. When looking at a checkered sheet with an uncorrected magnifying glass, the lines are no longer parallel, but curved. The magnifying glass which corrects this distortion is called “aplanatic”
We can clearly see in the top photo that the lines are curved and tighter in the center, while in the bottom photo, the lines are roughly parallel. Note that we will never have completely parallel lines and that the bottom magnifying glass having a larger diameter, this increases the deformations.
The second distortion that a good gemologist’s loupe must correct is the iridescent effect that can be seen on the edge of the field of vision. In fact, it is the decomposition of light, like a prism. A black line will appear with blue-green highlights. This problem is especially important when analyzing diamonds or colorless gems, because the eye should not be disturbed by flashes of color not coming from the stone itself. A magnifying glass correcting this effect is called “achromatic”.
It will be understood that we will need a magnifying glass that is both achromatic and aplanatic. The intrinsic quality of the glass used, the polishing of the lenses, the assembly of the lenses and the surface treatment contribute to the overall quality of the instrument. You will absolutely have to check your magnifying glass when buying, even if it is a well-established brand, because it may have defects, such as the following magnifying glass from Bausch & Lomb.
There are now two other factors to take into account when choosing our magnifying glass: the power and the diameter.
Let’s start with the simplest, which is the diameter. As a general rule, the lenses of gemologist’s loupes range from 17 to 21 mm. A 21mm diameter magnifier will be a little brighter than a 17mm magnifier, but that’s not obvious. When we analyze a stone, we only look in the middle of our magnifying glass. We move the stone and not the magnifying glass, so that our eye is always facing the center of the lens. If you have the choice between a 21 mm and 17 mm loupe of equivalent quality, you will have to test the one with which you are most comfortable, but the choice of diameter must be made last. So what is the difference? The price! A 21mm loupe costs more than a 17mm loupe.
Let’s see the power now. The custom is that in gemology we use a magnifying glass 10 times. This rule is used in particular for the gradation of diamonds for which a diamond not containing an inclusion visible with a 10X magnifying glass will be considered as having no inclusion. A diamond without inclusion visible to the naked eye but visible with a 10X magnifying glass will be considered as “very little included”, and so on. This is why it is important to have a 10X magnifier. Another aspect is the depth of field. This is the area of sharpness of the image seen through our magnifying glass. The higher the magnification, the lower the depth of field. Suppose that for a 10X magnifying glass it is 5 mm. for a 20X magnifier it would be 2 to 3 mm.We have the impression that with a 20X or 30X magnifying glass we will see the inclusions better but it is exactly the opposite. In fact, there is a strong risk of missing the inclusions because they will be blurry unless they fall right on them. It is a very well-known scam to offer a potential buyer to look at a stone with a 30X magnifying glass. We will hardly see anything. The one and only advantage of a 20X magnifier is that it makes it easier to identify an inclusion seen at 10X. A 30X magnifier is unnecessary because you literally have to stick the stone on the lens and you can’t see more than a few millimeters inside the stone.It is a very well-known scam to offer a potential buyer to look at a stone with a 30X magnifying glass. We will hardly see anything. The one and only advantage of a 20X magnifier is that it makes it easier to identify an inclusion seen at 10X. A 30X magnifier is unnecessary because you literally have to stick the stone on the lens and you can’t see more than a few millimeters inside the stone.It is a very well-known scam to offer a potential buyer to look at a stone with a 30X magnifying glass. We will hardly see anything. The one and only advantage of a 20X magnifier is that it makes it easier to identify an inclusion seen at 10X. A 30X magnifier is unnecessary because you literally have to stick the stone on the lens and you can’t see more than a few millimeters inside the stone.
Finally, a magnifying glass can have a hexagonal or round shape, be chrome or black. This is a matter of personal preference, although my preference is for black coatings which produce less reflections in the stone.
Now that we have seen the 4 physical criteria of a good magnifying glass, how do you choose it? By trying it! It should be aplanatic, achromatic, 10X wattage, and 17-21mm in diameter, but everything else is up to the user. You can have two loupes with identical criteria and not have the same ease of use. And regardless of the opinions of each other, you have to make your own opinion, magnifying glass in hand. A great magnifying glass for one might seem uncomfortable for another. However, there is one criterion to be taken into account: the price. Indeed, the price of a magnifying glass varies from 20 € to more than 500 €! Personally, I pay less attention to the price than to all the other criteria and especially to the comfort of use. My magnifying glass should not only give me confidence,but I’m also so used to using it (because it is very comfortable) that it has become a reflex to take it out of my pocket at the slightest opportunity. I bought a more famous model and I realized that I subconsciously used it a lot less, because I find it less pleasant. Suddenly, I returned to my old magnifying glass, which never leaves me. For information, my old magnifying glass cost me 23 €. Like all gemological material, I am convinced that we must set the right price at the start because a bad choice will force us to buy an instrument later. It is a false economy. There is also no need to rush to the very high end if a € 30 magnifying glass suits us better. As a general rule, a magnifying glass under $ 20 is a waste of money. Here are some examples of proven magnifiers:
Have you chosen your magnifying glass? Well done! We will now see how to use it.
The most important thing when looking at a stone with a magnifying glass is stability. It is absolutely necessary to have fixed points! The depth of field is so shallow that if the magnifying glass moves 2mm and the stone also moves 2mm, you can’t see anything. And 2mm freehand is really not much. To work around the problem, we use two support points. The first point is the face. The thumb of my hand holding the magnifying glass is placed on my face. Once you find a comfortable position, you don’t move! The position of the magnifying glass in relation to the eye never varies! We move the stone. You can move your head to direct the light. But we never move the magnifying glass.
Here I am using the magnifying glass case to hold it close to my eye. I might as well have held it around the lens. It’s a matter of personal preference. It’s up to you to try what suits you best. Note that my left arm rests lightly on the thorax for more stability.
Once you have found your comfortable position, you approach the stone with a magnifying glass. The hand holding the stone is resting on the hand holding the magnifying glass. The two hands being united, one can proceed to the examination of the stone in all peace and in a comfortable way. We always keep both eyes open to reduce fatigue. We can see in the two photos that I use my magnifying glass as a left-handed person, while I am right-handed. This is because my guiding eye is on the left and I am more comfortable there.
All that remains is to practice looking with your magnifying glass at everything that comes to hand: coins, hair on your hand (we all did it!), Tree leaves, etc. Remember that the sense of observation sharpens greatly with practice. I invite you to look at your stones frequently. If, like mine, they contain inclusions, you will find with experience that you will see things that you may not have noticed before. This is also what makes gemology interesting.
For further
- lighting
- tweezers
- darkfield loupe
- the polariscope
- the dichroscope
- the spectroscope
- Chelsea filter and other filters
- the refractometer
- the microscope
- UV lamp
- the carat balance