Felweyr is magic-infused tree sap, taken from trees that have tapped into a ley-line with their roots. It is incredibly toxic to anything capable of casting magic, making its handling a tricky matter. The fact that it usually only occurs in the depths of forests that are mostly left alone by mortals makes leaving it alone easy - but its value in enchanting and to a more limited extent in alchemy makes it too valuable to just leave it sitting in the woods.