Iron
You look at some gold and notice a fault in it. You scrape at the fault and see that it's just sprayed with gold paint. It's not gold, it's (comparatively) worthless iron!
You think you've found a ton of gold bars! Imagine being so happy about this. Then, imagine your heart dropping in your chest when you notice the fault. Then, it dropping down even farther when you figure out it's merely iron.
てつ
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Do you know that movie, Tetsuo (てつ) the Iron Man? We talked about it on Tofugu a while back. It’s one of the wackiest Japanese movies of all time. If you don’t know about Tetsuo the Iron Man, you should definitely look it up (if you aren't averse to horror/violent films). Tetsuo is one of the most insane movies in the history of ever.
Watch a trailer on YouTube for Tetsuo the Iron Man. Go ahead, do it now – I’ll wait. Did you watch it? Remember that nonsense every time you see this kanji.