It's called the Samulet as a combination of the name Sam and the word amulet.
It was borrowed by Castiel in the fifth season, as he was searching for God and it was supposed to burn white hot in God's presence. It didn't work and Castiel returned it in the episode "Dark Side Of The Moon."
Dean, for some reason not understood by fans, threw it away, despite obvious sentimental value.
The Samulet was supposed to burn in God's presence, but it turns out it was worthless.