Opal has one of the sharpest reputational reversals of any gem, and both ends of it are documented.
Pliny writes about opal with unusual enthusiasm, describing it as combining the fires of many stones (the flame of carbuncle, the purple of amethyst, the green of emerald), and tells the story of the senator Nonius, who was proscribed by Mark Antony and chose exile over surrendering a famous opal ring. Whatever the truth of the anecdote, the stone was highly valued in Rome.
Its reputation for bad luck is modern and traceable. The usual account attributes it to Walter Scott's novel Anne of Geierstein, published in 1829, in which the Lady Hermione wears an opal that flashes when she is agitated, is extinguished by holy water, and she is reduced to ashes. The book was widely read, and opal prices are reported to have fallen substantially in the following years.
That account is itself worth handling carefully. It is repeated in every gem book, it originates in George Kunz's The Curious Lore of Precious Stones of 1913, and the claim that the novel single-handedly crashed the market is an assertion by a well-informed writer who was also the chief gemmologist of Tiffany and had a professional interest in dismissing the superstition. The direction of the effect is plausible; the magnitude is not independently established. There is also a simpler contributing factor: opal is genuinely fragile and prone to crazing, and jewellers dealing with returned and damaged stones had their own reasons to spread a caution.
Opal is the October birthstone; pink tourmaline was added alongside it in 1952.