October

October's traditional birthstone is opal, while the modern list — added in 1912 by the Jewelers of America — names tourmaline instead, a completely different stone. Opal's shifting rainbow color, called play-of-color, is a real optical effect: microscopic silica spheres inside the stone diffract light into shifting flashes of hue. That beauty came bundled with a superstition of bad luck, a reputation traced partly to an 1829 Walter Scott novel featuring a cursed opal. Tourmaline, by contrast, occurs in almost every color a gem can be, which likely made it an appealing, less superstition-burdened alternative when the 1912 list was drawn up.

October also gets two birth flowers: marigold and cosmos. Marigold's warm orange and gold tones have long been linked to warmth and creativity. Cosmos takes its symbolism straight from its name — the Greek word kosmos means order and harmony, and the flower's neatly symmetrical petals are read as an emblem of both.