LXX uses κεφαλὴν γωνίας to translate לְרֹ֣אשׁ פִּנָּֽה (capstone [or keystone?]) in Psalm 118:22
(cf. ἀκρογωνιαῖον for פִּנַּ֤ת in Isaiah 28:16: ἀκρογωνιαῖος):
λίθον ὃν ἀπεδοκίμασαν οἱ οἰκοδομοῦντες οὗτος ἐγενήθη εἰς κεφαλὴν γωνίας (Psalm 117[118]:22 LXX)
The stone the builders rejected / has become the capstone (NIV).
Wikipedia: A capstone or coping stone is one of the finishing or protective stones that form the top of an exterior masonry wall or building (http://en.wikipedia.org/wiki/Capstone).
Wikipedia: A keystone is the architectural piece at the crown of a vault or arch and marks its apex, locking the other pieces into position (http://en.wikipedia.org/wiki/Keystone_(architecture)).