Perseus Dictionary Entry (Liddell and Scott [and Jones]'s Greek-English Lexicon, 9th ed., 1925-1940)
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LXX uses ἀκρογωνιαῖον to translate פִּנַּ֤ת (cornerstone) in Isaiah 28:16
(cf. κεφαλὴν γωνίας for לְרֹ֣אשׁ פִּנָּֽה (capstone) in Psalm 118:22: γωνία):
διὰ τοῦτο οὕτως λέγει κύριος ἰδοὺ ἐγὼ ἐμβαλῶ εἰς τὰ θεμέλια Σιων λίθον πολυτελῆ ἐκλεκτὸν ἀκρογωνιαῖον ἔντιμον εἰς τὰ θεμέλια αὐτῆς καὶ ὁ πιστεύων ἐπ’ αὐτῷ οὐ μὴ καταισχυνθῇ
Therefore, this is what the sovereign master, the Lord, says: /
Look, I am laying a stone in Zion, / an approved stone, / set in place as a precious cornerstone for the foundation. / The one who maintains his faith will not panic (NET).
The cornerstone concept is derived from the first stone set in the construction of a masonry foundation, important since all other stones will be set in reference to this stone, thus determining the position of the entire edifice (http://en.wikipedia.org/wiki/Cornerstone).
Occurrences in the GNT
Occurrences in the LXX