Simic Keyrune is a uncommon Artifact card from Gatecrash (GTC), collector number 237. Current market price: $0.24 (foil $0.39), TCGplayer market prices via Scryfall.
{T}: Add {G} or {U}. {G}{U}: This artifact becomes a 2/3 green and blue Crab artifact creature with hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)