Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What was the U.S birthstone in the 15th-20th century when in 2013 it was amethyst?

### Inputs:
table_name: table, columns: ['Month', '15th- 20th century', 'U.S. (1912)', 'U.S. (2013)', 'Britain (2013)', 'Hindu']

### Response:
SELECT 15th- 20th century FROM table WHERE U.S. (2013) = amethyst