A jewelers loupe (pronounced loop) is a small magnifying glass that folds into a metal protective case. It is used to view small jewelry pieces and gems to detect flaws and to read marks that can't be seen using normal vision. There are several characteristics to consider when purchasing a jewelers loupe depending on individual needs.

The strength of the lens is described by a number followed by the letter "x" which stands for "times". For example 10x, 20x, 30x. The diameter of the lens is shown in millimeters, 12mm, 18mm, 21mm, etc. And the lens itself consists of either one, two, or three or more lenses built into the assembly. The more lenses in the construction, the better the quality. A lens will commonly be called a doublet, triplet, and so on.

Some of the things to look for in a quality lens are optical quality glass, a roll pin in the hinge, and a leather case. Generally lower quality loupes will be made from window glass with screws in the hinge, and have a plastic case. This is not always true, but in most cases if you look for a name brand product and pay attention to the features I've covered above the loupe you purchase will be a good one.

The proper way to use a jewelers loupe is straightforward and easy. Start with a well lit room or natural sunlight and a place to sit down so your hands will be steady. Make sure to clean the piece of jewelry and the lens with a soft cloth to remove surface dirt before viewing. Also put a cloth underneath the work area to keep items from rolling away if you drop them.

Expand the loupe and put your index finger through the metal case. Position the lens so that your thumb is above the case towards your face, and rest your thumb on your cheek so that the loupe is about an inch away from your eye. Generally if you are right handed you will hold the device in that hand, and vice versa.

You can keep both eyes open or close one eye as long as it doesn't cause eye strain. Hold the jewelry item with a pair of tweezers in your spare hand, and move it closer or farther away from your eye to bring it into focus. Keep the loupe stationary. A gemstone should be viewed on all facets of the stone to examine every angle. Look for dark inclusions inside the stone and any flaws in the cut of the facets.

If you are casual collector or a dealer in vintage jewelry it's good to become familiar with using a jewelers loupe correctly. It takes a little practice but once you become proficient you will be able to identify valuable gemstones, maker's hallmarks, and collectible jewelry.