The most frequently used type of jewel in a watch is a hole jewel. These are doughnut shaped to fit over the ends of the wheel arbors (gear axels) and are designed so they draw lubricant in towards the arbor where it's needed. Cap jewels are the 2nd most common jewel and look like frisbees. They cover the ends of the arbor and its corresponding hole jewel. Pallet jewels have a trapezoidal brick form and are located at the end of the pallet that engages the escape wheel. On the opposite end, where the pallet fork contacts the balance arbor, is the roller jewel which looks like a half a can of cranberry sauce sliced top to bottom. ......hole jewel ......... cap jewel .........pallet jewel ..........roller jewel............ image from elginwatches.org A basic escapement and balance assembly use 7 primary jewels - 1 roller jewel, 2 pallet jewels, with 2 hole jewels and 2 cap jewels at the top & bottom of the balance arbor. The gear train employs 8 hole jewels and the center wheel has 2 bringing the total to 17. Up until almost 1900, even high grade railroad watches did not jewel the center wheel. Since the center wheel moves so slowly it doesn't create a great deal of friction, and over-jeweling a watch was perceived as making it less rugged. The advent of synthetic jewels made the argument moot since it was so much less expensive to include these 2 jewels and the man-made versions were much more durable. An additional 2 - 4 cap jewels may be added to the escape wheel and/or pallet; 2 - 4 more hole and/or cap jewels can be utilized on the mainspring barrel as well. Several extra jewels can be installed in an automatic winding movement to maximize winding efficiency. Each additional feature may require extra jewels, so watches with many complications can have over 40 functioning jewels. Breguet experimented with shock protection in the late 1700's. Since then there have been several methods invented including Novachoc, Kif, Unisafe, and Parachoc but the most commonly used now is Incabloc. Introduced in 1933 by Universal Escapements of Switzerland, it was not registered as a trademark until 1957. Incabloc When a watch is dropped or banged against something, the balance assembly is the area that takes the most damage. The balance staff can bang into the cap jewel at its end. This often led to broken balance pivots or cracked jewels at the point they connect. Instead of firmly mounting the cap jewel in place, Incabloc holds it in position with a small retaining spring that absorbs the shock by letting the parts move, which keep the pieces intact. You can see it in the photo above - it's shaped like the end of a bottle opener. After the impact is absorbed, the spring returns the pieces to their original positions.