A jewel is a valuable gem or stone, one that might be set in a ring or necklace. Your grandmother might keep her jewels locked in a safe, to protect them from thieves.
You can see large, gorgeous jewels in museums and jewelry stores. The diamond in a diamond ring is a jewel, and the rubies in a pair of sparkly red earrings are also jewels. You can also use the word to mean "person as special as a jewel." The Old French root is jouel, which may stem from jocus, "sport or pastime" in Latin, and "that which causes joy" in Vulgar Latin.