Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_34 (us__2013_ VARCHAR)
### Question:
What was the U.S birthstone in the 15th-20th century when in 2013 it was amethyst?
### Answer:
SELECT 15 AS th__20th_century FROM table_name_34 WHERE us__2013_ = "amethyst"