Before you buy a magnifier...
After you buy your rock hammer and map out what geological place you will want to explore, make sure to pack your rock magnifier loupe or jewelers loupe. You will want a lightweight, powerful magnifier loupe that has impeccable optics and is easy to use. Get the best magnifier for demanding jobs like inspecting gems; in the field, for quick looks at minerals, buy a decent magnifier you can afford to lose...
How to Use a Magnifier
Hold the loupe up next to your eye, then bring your specimen close to it, only an inch from your face. The point is to focus your attention through the lens, the same way you look through eyeglasses. If you normally wear glasses, you may want to keep them on. A magnifier won't correct for astigmatism.
How Many X (Power)?
The X factor of a magnifier refers to how much it magnifies. Geologists (and Numismatists) like to have 5x to 10x, but more than that is hard to use in the field because the lenses are very small. 5x or 7x lenses offer a wider field of vision, while a 10x magnifier gives you the closest look at tiny crystals, trace minerals, grain surfaces, and microfossils. Most people find that a 10x loupe is just the right level of magnification to provide the best magnification, light capture, and depth of field. Given the same X factor, a larger lens of course is better. So is a ring or loop to attach a lanyard, as well as a leather or plastic case to keep the dust off.
Magnifier Flaws to Watch For
Check the lens for scratches. Set the magnifier loupe on a piece of white paper and see if the lens adds color of its own. Now pick it up and examine several objects, including one with a fine pattern like a halftone picture. The view through the lens should be clear as air with no internal reflections. Highlights should be crisp and brilliant, with no colored fringes (that is, the lens should be achromatic). A flat object should not look warped or buckled— move it around to be sure. Good lensmakers combine three pieces of glass to correct for chromatic aberration— what gives an image blurred, colored fringes. Doublets (two pieces can be satisfactory, but the triplet is the gold standard.
Triplets are made of three (3) lenses. Every lens refracts (bends) the light passing through the glass. Different wavelengths (color) of light is bent a different amount. The combination of the 3 lenses bonded together to form one lens are engineered so that the light coming into the lens is corrected so that the light coming out of the lens is color correct, or the lens is achromatic correct. The triplet lens is also aplanatic meaning that the lens is corrected for spherical aberration making the image clear and in focus from the center to the edges.
Folding magnifiers, magnifiers made of of multiple lenses that fold in and out of a housing are handy for providing simple magnification at multiple levels but each lens and the air gap between each lens refracts the wavelengths a different amount. The image is distorted across the lens and the color you see are not the same as those of the object you're viewing.
Many of the cheap loupes available on the market are simply made of multiple magnifying lenses stacked on top of each other in a metal barrel. The result is gross image and color distortion, but at a very cheap price.