I was going to translate a phrase into English. The phrase talks about how doing something changes one's characteristics and material. like if you do such and such a thing, all your sensitiveness, anger, irritability, ... will fade away and you become a calmer and happier person. (a permanent change) So, the writer was using a word that would literally translate as "material" for such a context. but, is the word "material" really usable for human beings? in other words, can we say, if you do such and such a thing, your material will change and you become a better person?
It's late (time for bed, so please excuse any typos, etc.!), but very quickly you would say 'character' not 'material'. 'Mettle' refers to the amount of resolve someone has to complete a task, or endure a hardship. Example: "Let's test this new fellows's mettle, and see if can stick to the task until the end." So, in this case the 'new fellow's' mettle (being tested) is an indication of the quality of his character. If he completes the task then he is of capable (good) character (at least for this particular task being asked to do). But 'material' or testing someone's 'material' would never be a proper usage with regards to people, but rather things, e.g. "What kind of material is this machine/fabric/tool/etc., etc. made of, and is it strong/weatherproof etc., material?"