Things that are faceted have many different sides or faces. The sparkling ruby in your grandmother's necklace is faceted.
The adjective faceted is most often used to describe a gem or semi-precious stone that's been cut and polished. A diamond ring gleams and sparkles in part because it's faceted, giving it many small surfaces off of which light can reflect. Things that are figuratively faceted have a lot of different features or attributes: "It's impossible to see how faceted someone is when you first meet them." Faceted comes from the French facette, "little face."