Loupes- Introduction
This article has been around for a few years, but there are still some basic considerations when obtaining a loupe. 
The ideal magnification for a mineral collector's loupe (pronounced "loop") is 10x.  That's ten power, or a magnification of ten times actual size.  Decent 15x and 20x loupes are more expensive and smaller in field of view than the 10x ones. Usually you have to put the 20x loupe (and your face) so close to the specimen that it blocks most of the light.  It's self-defeating.  Lens quality also makes a much more noticeable difference at these higher magnifications. 
From here on in I'll be referring mostly to loupes of the 10x variety, unless otherwise stated.   We will also take a brief look at the 14x-20x category.
First of all, not all loupes are good. Without going beyond the scope of this website, I'll just say that the simple, single-element lens used in "no-name" loupes has inherent problems with image distortion, blurriness, or distracting red & blue halos. Sometimes you get lucky and end up with a single lens that gives a good image- but you can't rely on that.  (As we'll see later, there are ways around this.)
To compensate for optical aberrations and make consistently good lenses, manufacturers have resorted to multiple lens elements in one loupe (usually three, called a "triplet"), or sometimes a single lens which is thick and has a special groove around the circumference to give a similar corrective effect (this special lens is called a "Coddington").

Varying Prices & Qualities

Inexpensive, single-element loupes can be had for as little as US $3 to $5, especially if you buy multiple units.  Some of these are essentially junk, while others are pretty good. It's luck of the draw- but low quality predominates. The majority are unmarked, so it's difficult to tell a good one from a piece of junk without actually peering through the lens.
The classic, 3-lensed triplet loupes range from US $40-$75.  These include the Belomo and the Hastings Triplet (from Bausch & Lomb).  I like the Belomo better, but some people really love their Hastings.  Other manufacturers such as Nikon produce great loupes, but the prices are $75 and up.  I can think of at least one make that's in the $300 range.
In between the previous two types is the Coddington. This is actually closer in price and quality to the triplet, and for many applications will provide nearly equal results. Bausch & Lomb even manufactures one that has built-in illumination... very useful.
Less frequently encountered is the "doublet", so called because it uses two lenses instead of three. Doublets are sometimes offered by optical companies as "affordable alternatives" to their triplet models. Some doublets actually have a Coddington lens plus a regular lens... I haven't tried one of these, but I'd expect the image quality to be very good.  Generally, doublets are going to be superior to any single-element spheric  lens.
There are also loupes with multiple but independent lenses. What this means is that you can choose one lens of 5x, 7x, 9x, etc... or a combination of 2 or 3 lenses to give yet higher magnifications. The individual lenses can be handy, but using them together to get higher magnification is never quite as good as using an actual triplet loupe.  Bausch & Lomb makes one of these, and it's not bad, but I actually use a  cheap, no-name China / India loupe in preference to it most of the time.  You may have a different opinion, but I just don't feel like dealing with three separate lenses that have to be lined up perfectly.

Not all so-called "triplet" loupes are true 3-lens magnifiers. The type at right is often advertised as a "triplet", yet disassembly reveals it to have only one, simple lens.
Some single-lens "fake triplets" actually give very good image quality, but this appears to be luck. The quality is unpredictable but usually not that great. I keep these around for field-collecting, unless the image quality is really bad.
a "fake triplet"- one lens!
Some Reviews!

Since I couldn't hope to do an exhaustive review of every loupe you could possibly encounter, my list is somewhat arbitrary. However, I think it includes enough variety to prove useful. I've done some hunting for loupes on websites and eBay auctions. There are some pretty decent loupes to be had on eBay, even in the single-lens "fake triplet" category (see below). I bought a few of these way-back-when at $3 each and have found some of them to be pretty decent, others not (see below).  Today, the cheapest price for these "no-name" loupes is more than $3, but they're still not expensive.

Bausch & Lomb- 10x (Hastings Triplet); nickel-plated or gold-colored teardrop-shaped enclosure;
PRICE: medium to high ($50-$75);
QUALITY: excellent.
Compound, 3-element lens; among the best available.

Bausch & Lomb- 10x (Illuminated Coddington); penlight-like metal barrel holds two AA batteries and has switch; no pivot- lens is at end of metal barrel...
PRICE: medium (about $30-$40);
QUALITY: very good to excellent.
Lens is single-element but has specially-incised channel to add correction. With a brand name such as B & L you can be sure the optical quality is consistently high.  Note: illuminator bulb seems to burn out easily but is replaceable.

Belomo- 10x;  21 mm lens;  marked "Made in Belarus";  black enclosure of unusual shape;  pivot has small Phillips screws.
PRICE:  medium (about $35-$40);
QUALITY:  excellent.
Lens is composed of three, joined elements. I've tested dozens of these; it appears their quality is pretty much consistent no matter how many I test.  It also has a bigger field of view than the Bausch & Lomb Hastings.   Even at 40 bucks, the Belomo's quality is so good that it's worth it.

Schneider- 10x; hexagonal enclosure;
PRICE:  so high that it crashed my web browser ($325);
QUALITY:  unknown;
I wasn't able to afford an example for testing. If you're going to spend $300+ on a single item for this hobby, put it toward a decent stereo inspection microscope. In fact, you shouldn't spend much over $50 for a top-quality loupe unless your job is to grade diamonds all day.  You could still probably do the job just fine with a Belomo or a Bausch & Lomb, but then again I'm not a diamond grader so I wouldn't know for sure.

"SE Brand"- 10x; 17 mm lens; chrome plated, teardrop-shaped enclosure; pivot has small Phillips screws....
PRICE:  lowest
QUALITY:  varies;
The one I tested had surprisingly low spherical aberration. I ordered an additional 10 of these to test.  Eight of them were quite good for the price, while two had weird blurring near the centers (ridiculous!!), making them just about unusable.  Not bad overall- nine keepers out of 11 tested units.  Who knows whether another sample lot would have the same number of keepers, though.  That's the problem;  the quality control is pretty much random.

"SE Brand"- 10x; 21 mm lens; chrome plated, diamond-shaped enclosure; pivot has small Phillips screws....
PRICE: very low
QUALITY: varies;
Single lens element.  Out of four I tested, one of them was surprisingly good. The other three were not that great, but I guess they were adequate for field collecting.  That's really the point of these loupes.  If you accidentally leave it on a rock somewhere, it's no great loss. 

"Triplet 20x"- 20x; 21 mm lens;  black, hexagonal enclosure
PRICE:  low to medium
QUALITY: decent;
The lens is definitely a triplet / compound setup. The high magnification gives very, very shallow depth of field. There is still some spherical aberration despite the multiple lenses. You also have to hold the thing very close to the item you're planning to view... so close that it blocks out most of your light.  This is not a problem with the 10x, just the 20x version.  I think 14 to 16x is as high as I'd recommend for a handheld loupe, especially for microminerals.  The shallower depth of field at 20x is alright for tiny crystals, but the real drawback is that you're so close to the specimen that it's blocking out most of the light.

the Belomo 10x
The winner is still ... BELOMO  for overall quality vs. price.  

 Shown here with some frayed green string that I've used to wear it around the neck. The string was once day-glow green, but dirt from the Buckwheat has faded it quite a bit. 

A drop of Bond 527 cement holds the knot in place.  Most of the time, I no longer cart my treasured Belomo into the field, though... the cheap single-element job goes along now to suffer the scratches and dings of flying rock chips.  However, if you're a serious micro collector and plan to spend the whole day peering at tiny crystals, the Belomo's lens quality will be more relaxing on your eyes.

Closing Thoughts

There are several different types and qualities available on the market for loupes.  Even at $40, the Belomo is still the best thing for the money.  However, due to the rough treatment that any real field-collector gives to a loupe, you might also consider a cheaper model for field use.   Here's another big reason:  if you're into micromounting or micromineral collecting, the loupe is mostly just a sorting tool.  Once the loupe tells you a specimen is worth further study, it gets viewed under a stereo microscope.   So, spend $10-$25 on a loupe and save the rest for the microscope.   Or, get a Belomo and a "cheapo" and keep the Belomo for when you visit mineral shows and symposia.
Earlier I said there was a way around the cheap quality of "singlet" loupes.  These bogus "triplet" loupes, which really have only one lens element, have a tendency for poor quality-control.  However, if you deal with a seller who culls out any obviously bad ones before even selling them to you, it works out alright.  That means a bit of loss of inventory, so it obviously drives the price up a bit.  However, that still puts it in the under-$15 range (according to 2012 prices). 
I happen to know such a dealer who inspects each one prior to shipping to make sure it's usable.  Twelve bucks, and you've got a passable loupe for field use.   And hey, if you leave it sitting on a rock, at least it wasn't a forty-dollar one.   Then again, my Belomo has accompanied me to dozens (if not hundreds) of collecting trips, and it's still intact.

the Buckwheat Dump To avoid losing that  magnifier, wear it around your neck for safe keeping.  I used string, but you can buy lanyards from some dealers.
At leftThe pile of rocks that ate one Bausch & Lomb Lenscope, a prybar, 2 or 3 chisels, several gloves, an Estwing crack hammer, and at least three sets of goggles. Franklin, NJ.

Don't set your loupe on a rock and then think you're going to come back and find it later.

I hope you've enjoyed this article.  Thanks for stopping by!

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