Which month in 2013 did the U.S. use the ruby birthstone?
CREATE TABLE "birthstones_by_cultures" (
  "month" text,
  "15th_20th_century" text,
  "u_s_1912" text,
  "u_s_2013" text,
  "britain_2013" text,
  "hindu" text
);
SELECT "month" FROM "birthstones_by_cultures" WHERE "u_s_2013"='ruby';