Its basically a metal (I think) that you can get your dog/Sim to dig up. If you had a dog, you might find some if their digging skill is high, I started finding some around lv 6 in digging, its not too hard to find if your dog has the digger trait, since they often automatically go digging when their needs are met, so you might find some, and not notice your dog was out digging (also helps to have the Treasure Hunter lifetime reward for your dog, since you can find 2 things of the same category, like metals or seeds, in 1 hole while digging, this also helps when searching for relics, it is expensive though!)