Table: birthstones
Columns: month
15th_20th_century
u_s_1912
u_s_2013
britain_2013
hindu

Question: What was Britain's (2013) birthstone when the U.S (2013) birthstone was ruby?
SQL: SELECT "britain_2013" FROM birthstones WHERE "u_s_2013" = 'ruby'

Question: What was the U.S birthstone in 1912 when in 2013 the birthstone was amethyst?
SQL: SELECT "u_s_1912" FROM birthstones WHERE "u_s_2013" = 'amethyst'

Question: What was the U.S birthstone in the 15th-20th century when in 2013 it was amethyst?
SQL: SELECT "15th_20th_century" FROM birthstones WHERE "u_s_2013" = 'amethyst'

Question: Which month in 2013 did the U.S. use the ruby birthstone?
SQL: SELECT "month" FROM birthstones WHERE "u_s_2013" = 'ruby'

Question: In 1912 when the U.S used sapphire, what was the Hindu birthstone?
SQL: SELECT "hindu" FROM birthstones WHERE "u_s_1912" = 'sapphire'