John is a different sort of significant. Sherlock- and others- have to keep this in mind.
Gold and Iron
John wasn't stupid; many forgot that when he was around Sherlock because his flatmate was brilliant; a kind of brilliant that made John's own talents seem simple in comparison. Admittedly, he was rather unremarkable compared to Sherlock, but he wasn't useless, nor was he unreceptive.
Quietly, as Sherlock puzzled over a piece of the puzzle that he seemed to be missing, John reminded him to consider human habit and conformist situations. Sherlock's eyes lit up and he took his train of thought on a new track.
If Sherlock was gold, John was iron- not as pretty but just as valuable.
Prompt was #20: gold