Orcrest is a stone of power.
One was found by Trell and used to end a drought which afflicted The South Plains.
Upon Thomas Covenant's fourth trip to the land, 10 years after his first defeat of Lord Foul, (several millenia in the Land's time), the Graveler of Mithil Stonedown named Sunder, son of Nassic, also possesses a piece of orcrest, which has since become known as sunstone. Gravelers use the power of the sunstone and blood to shape the Sunbane to allow water to flow, crops to grow, etc. Orcrest can activate the white gold, although Covenant is not sure how.