Fobs, a strange word! Generally a fob is an item designed to hang from a watch chain as decoration or balance.
Really almost anything can be a fob normally its something that means a memory to the owner often a sporting item or club.
Fob seals, these are seals contained in normally a decorative fob i.e. a metal surround normally of gold or silver sometimes with animals etc, and contained in the base a seal, these are normally a love token or cryptic notes like a dove with a letter or a twig or similar, or of course you're favourite greyhound or horse, or more likely you're family crest. Lastly just a simple monogram , except anything but simple normally.
The seals themselves are occasionally glass, but mostly lapis lazuli, carnelian as it is hard and carves well, rock crystal,bloodstone, black onyx . A great field to collect and more and more being worn as jewellery in its own right as pendants and charms.