Rock

You will keep in perfect peace those whose minds are steadfast, because they trust in You. 
Trust in the LORD forever, for the LORD Himself, is the rock eternal. 

Isaiah 26 v 3, 4

Steeadfast - samak : to lean, lay, rest, support, lean upon

Keep - natsar : to guard, watch over, preserve

Peace - shalom : completeness, soundness, welfare, peace

Trust - batach : to have confidence in, trust in