CREATE TABLE table_name_34 (us__2013_ VARCHAR2)

What was the U.S birthstone in the 15th-20th century when in 2013 it was amethyst?
SELECT 15 AS th__20th_century FROM table_name_34 WHERE us__2013_ = "amethyst"
SELECT 15 AS "th__20th_century" FROM "table_name_34" WHERE "amethyst" = "us__2013_"