There are few things more terrifying for a flying or bug pokemon than a stone which floats in the air, escaping their notice until a high-speed frontal collision. The means of doing this, as a technique, completely transformed the pokemon league, bringing low even the once-mighty Charizard; the thought that this might also come in the form of a pokemon was too terrifying for many a Bird Keeper or Bug Catcher to imagine.
Glimmet do not, in truth, live up to this menacing reputation; they are stealthy because they are small and unevolved, and one is not likely to encounter them except when spelunking. Zubat fly ably around them, noticing them through their echolocation, and any disoriented birds who actually fly into their habitat likely have bigger problems. Reports of Glimmet growing on the outside of caves, and causing birds nesting on mountains to crash, or Ponyta to trip and fall hundreds of feet to their deaths, are likely nothing more than horrifying legends.
Glimmet are not (ordinarily) ghost pokemon, and do not only stalk nightmares. Scientists have increasingly come to link Glimmet to the process of Terastallization, a strange process in Paldea by which any pokemon can overcome their weaknesses… in exchange for picking up new ones, along with the other characteristics of whatever type it becomes.
There is intriguing anatomical evidence that Glimmet have themselves, at some point in the past, undergone this process. Although there is no doubting Glimmet's crystal structure, it has a noted physical similarity with Lileep, which only became a rock as the result of fossilization. Glimmet's body, that is to say, resembles that of grass pokemon; the very kingdom on which their enemies feast!