Frisk's bracelet, now in Aliza's possession, was a gift to her from Sans, a token to mark the mating of their souls. It is composed of a metal unknown to humankind, as it fell as a small meteor into the Underground from space, later smelted into fine chain, and upon it hangs a large sapphire heart, glowing with a reserve of Sans' magic. The meteorite was once Sans' pet rock, and if you try very, very hard, the bracelet occasionally seems to smell like sprinkles.