iiiii was kinda surprised about that statement, so i decided to check up on it, and im not seeing any significant or reliable info on the internet about malachite being toxic when wet. this page lists everything from chemical composition to how it reacts to heat, acid and water, and under “health risks” it says “no information about health risks,” which sounds inconclusive until you realize this website is exhaustively researched and says the same thing about quartz.
the only page that even mentions malachite’s toxicity (aside from “crystal healing” sites, tumblr posts, or steven universe fanpages) is on a pesticide registry, where it lists symptoms, not of malachite poisoning, but of copper compound poisoning in general. malachite is very slightly soluble in water, and can release small amounts of copper sulphate, but copper sulphate is also commonly used as a fungicide in swimming pools and hot tubs. it is generally considered safe in reasonable amounts.
i was wondering where this misconception came from, so i did more searching for info about “malachite poisoning,” and one website that regurgitated the same weird infaux about it mentioned something about it being made into paint. malachite was used as a pigment in antiquity, but was replaced in the 1800s by malachite green, a synthetic dye named after the mineral but not actually derived from it at all. it’s toxic, but its toxicity has pretty much nothing to do with whether or not it’s dissolved in water.
the specific story on that website sounded pretty similar to a completely different story: scheele’s green, a synthetic dye produced from arsenic. arsenical wallpaper was a major problem in the 1800s, and was more dangerous when damp.
in short: no, wet malachite does not produce “toxic fumes,” and it didn’t kill napoleon, that was arsenic.