user: Question: What was the U.S birthstone in the 15th-20th century when in 2013 it 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 "15th- 20th century" FROM "Table" WHERE "U.S. (2013)" = 'amethyst';