A jewel bearing is a bearing made from a jewel, usually corundum (this includes both sapphires and rubies), and, in modern times, synthetic corundum. In theory, diamonds would make the best bearings, but cost prevents this from being practical. Glass bearings may also be referred to as 'jewel' bearings. They are not as strong, or as classy, but they do have very low friction coefficients, are very cheap, and in many applications they work just as well as a traditional jewel bearing.
Jewel bearings are best known for their use in constructing watches, where the quality of a watch can be gauged by the number of jewels that it has. (It should be noted that a basic watch only needs 15 to 17 jewels, and while features such as self-winding and shock resistance can get the number up to about 30-40 jewels, more than that is just bling). Not all of a watch's jewels are bearings, but a good portion are. Other watch jewels include pallet stones and the impulse jewel in the escapement.
Jewel bearings have a low friction coefficient and breakaway torque, require little or no lubrication, will not deform under heat, can be very small and highly precise, and with the advent of synthetic sapphires they are not even particularly expensive. They are very low maintenance, and generally have very long lives. As jewel bearings are crystalline (and therefor brittle), they are used almost exclusively in low load, low vibration, and low speed mechanical instruments, such as watches, clocks, compasses, and various optical, navigational, and medical instruments. You will rarely, if ever, see a jewel bearing designed to bear a load of more than 500 grams, or larger than 10 mm in diameter. They are often less than one mm in diameter, and not designed to support even one gram.
Here are a few of the more common types of jewel bearings:
The Ring bearing: This is simply a bearing in the shape of a ring, with a rotating shaft passing through the hole. The shaft is not usually a jewel, but rather some sort of metal. Unless otherwise specified, this is probably what is referred to when you hear someone talking about jewel bearings.
There are many specific terms used to describe ring bearings, the most common being the basic straight hole rings, a flat disk with hole through it, and the olive hole rings, in which the interior of the bore has a convex curve, giving the bore an hourglass shape. The olive hole reduces the surface area of the contact points (to just one point on the curve, rather than the entire length of the bore), thus reducing friction. The front and back surfaces of the bearing are often flat, but a rounded, convex surface is not uncommon, and is referred to as bombe.
The V-bearing: AKA a vee bearing. A V-bearing is (usually) a small disk with a conical indentation. A shaft is set standing with its pointed end resting in the indentation; this gives the shaft a very small point of contact, and thus very low friction. The V-bearing itself provides support in only one direction, and will need other bearings to prevent lateral motion. Obviously, this design works the best when the shaft is standing vertically on the bearing (or in the case of anemometers and current speed indicators, standing against the currant). If the bearing is mounted horizontally there must be a very small (0.003 to 0.005-in.) gap between the point of the shaft and the bearing to prevent uneven rolling against the side of the cone. Hard glass V-bearings can have so little friction that some compass manufactures add a small amount of lubricant to increase friction, as otherwise the compass needle could osculate back and forth indefinitely. V-bearings are also used in galvanometers and magnetic meters.
A Cup bearing is a form of V-bearing in which the indentation is a shallow semicircular concavity, which may be used in conjunction with a ball bearing or other rounded pivot.
An end stone (AKA an endstone or a cap stone) is generally simply a flat disk, although one side may be curved (for example, in a incabloc system). Their primary function is as a simple thrust bearing, in which two jewels (generally the endstone and a ring bearing) are set against each other, with some sort of oil as lubrication. A shaft passes through the ring bearing and rests against the end stone. As the shaft is already braced to sit on a specific point by the ring bearing, a V-bearing is not needed.
http://www.birdprecision.com/bearings/ring.cfm (images; different types of jewel bearings)
http://www.info-uhren.de/technik/techn/stosssicherung.html (image; ring stone, end stone)
http://elginwatches.org/help/watch_jewels.html (images of the jewels in a watch)