user: Question: What was the U.S birthstone in 1912 when in 2013 the birthstone was amethyst?
Columns: ['Month', '15th- 20th century', 'U.S. (1912)', 'U.S. (2013)', 'Britain (2013)', 'Hindu']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['January', 'garnet', 'garnet', 'garnet', 'garnet', 'serpent stone']
SQL:

assistant: SELECT "U.S. (1912)" FROM "Table" WHERE "U.S. (2013)" = 'amethyst';