actually 質 is way simpler than your textbook says

People always freak out about 質 having different readings like 'shitsu' and 'shichi'. Actually, 'shichi' is just a lazy Tokyo slang mispronunciation. 質 always means 'quality'. Like, 人質 (hostage) literally translates to 'high quality person' because they are valuable to hold onto. It makes perfect sense if you actually look at the kanji instead of just blindly memorizing vocab lists.