Kryptonite key

From Smallville Wiki

"The composition was unique. There's nothing like it on Earth, with one exception. The meteor rocks found here in Smallville. So I have fashioned my own key. It's identical in every detail." - Lionel Luthor, to Clark Kent, Exodus

The kryptonite key, identical to the Key, was created by Lionel Luthor to put into the cave wall. He created it out of kryptonite because he said that the original key was made out of a metal alloy close in composition to the meteor fragments.

Clark Kent stole the kryptonite key to destroy his ship. Once he placed the copy into the slot where the real key was supposed to be, a viral growth appeared on the exterior of Clark's ship as it floated in the air. The ship exploded, causing an energy wave that triggered Martha Kent's miscarriage of her unborn child.

The kryptonite key was destroyed along with the ship.