Amethyst dragons featured pretty heavily alongside the other gem dragons in Fizban's Treasury of Dragons, which IIRC was a popular book in the 5e line-up.
Yeah but they are not in the Monster Manual, they are not in any novels that I know of, they are not in any of the Forgotten-Realms specific stuff, they are not in any published adventures. I would wager that fewer than 10% of players even know what they are.