Theory on the law of Eywa
\[WARNING HEAVY OVERTHINKING AHEAD\]
(Also feel free to fact check the science. I looked up as much as I could, but it’s not like I’m an expert)
Throughout the series, through the mouth of many Na'vi; it is stated that metal poisons the heart and corrupts the soul.
Fun fact, Na'vi bones are composed of naturally occuring Carbon fiber as stated in the first film.
Another fun fact, Carbon fiber, when put in contact with certain metals and electrolytes can cause something called galvanic corrosion.
On it’s own, Carbon Fiber can be fairly conductive to a medium degree and can’t really cause problems to a living entity. It’s why humans use it as artificial bone replacement.
Carbon Fiber causes this galvanic corrosion in other metals. So let’s say a Na'vi carved an arrow tip out of Unobtanium, a ionized super conductor that contains electrolytes. If effectively shot and landed; Would it leave traces of material that could then be caused to corrode on contact with Na'vi bones and travel through the bloodstream and by extension literally poison the heart?
Either way I always thought that the choice of making Na'vi bones Carbon Fiber sounded arbitrary. The fact that Carbon Fiber can damage metal either feels like a happy accident, or some sort of 4D chess.