CREATE TABLE table_62328 ("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 1912 when in 2013 the birthstone was amethyst?
SELECT "U.S. (1912)" FROM table_62328 WHERE "U.S. (2013)" = 'amethyst'
SELECT "U.S. (1912)" FROM "table_62328" WHERE "U.S. (2013)" = 'amethyst'