After you get a rock hammer—maybe even before—you'll need a magnifier. The big Sherlock Holmes type lens is a cliché; instead you want a lightweight, powerful magnifier (also called a loupe) that has impeccable optics and is easy to use. Get the best magnifier for demanding jobs like inspecting gemstones and crystals; in the field, for quick looks at minerals, buy a decent magnifier you can afford to lose.
Using a Magnifier
Hold the lens up next to your eye, then bring your specimen close to it, only a few centimeters 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?
The X factor of a magnifier refers to how much it magnifies. Sherlock's magnifying glass makes things look 2 or 3 times bigger; that is, it's 2x or 3x. Geologists 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.
Magnifier Flaws to Watch For
Check the lens for scratches. Set the magnifier 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 to and fro to be sure. A magnifier should not be loosely put together.
Given the same X factor, a larger lens is better. A ring or loop to attach a lanyard is a good thing; so is a leather or plastic case. A lens held with a removable retaining ring can be taken out for cleaning. And a brand name on the magnifier, while not always a guarantee of quality, means you can contact the manufacturer.
Doublet, Triplet, Coddington
Good lensmakers combine two or three pieces of glass to correct for chromatic aberration—what gives an image blurred, colored fringes. Doublets can be quite satisfactory, but the triplet is the gold standard. Coddington lenses employ a deep cut inside the solid glass, using an air gap to create the same effect as a triplet. Being solid glass, they cannot ever come apart—a consideration if you get wet a lot.