CREATE TABLE table_62329 ("Month" TEXT, "15th- 20th century" TEXT, "U.S.  ( 1912 ) " TEXT, "U.S.  ( 2013 ) " TEXT, "Britain  ( 2013 ) " TEXT, "Hindu" TEXT)

What was the U.S birthstone in the 15th-20th century when in 2013 it was amethyst?
SELECT "15th- 20th century" FROM table_62329 WHERE "U.S. (2013)" = 'amethyst'
SELECT "15th- 20th century" FROM "table_62329" WHERE "U.S. (2013)" = 'amethyst'