heart of stone
A very cold and unfeeling nature, as in You'll get no sympathy from her; she has a heart of stone. This idea dates from ancient times and in English appeared in the Bible (Job 41:24). [Early 1600s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.