It’s one of the silliest party tricks in fiction. A contestant appears at the door of 221B Baker Street, and the great detective observes, “Beyond the obvious facts that he has at some time done manual labour, that he takes snuff, that he is a Freemason, that he has been in China, and that he has done a considerable amount of writing lately, I can deduce nothing else.”
This appears to be a brilliant act of deduction, but Holmes waves it away. The client’s right hand is bigger than his left hand, which is apparently a thing that happens when you do manual labor; he’s wearing a Freemason tiepin, which is apparently a thing that you wear when you’re a Freemason; his shirt cuff is shiny, which is apparently a thing that happens when you do a considerable amount of writing lately; and he’s wearing a T-shirt that says Spring Break China 1891.
These are just sound effects, obviously, because it’s all a setup. Arthur Conan Doyle deliberately festoons these chumps with splashes of mud from a specific area where the clay is a unique shade of ochre, just to impress us with Sherlock’s amazing ability to deduce things from the clues that Doyle put there to be deducted upon. The author already knows where the treasure is; all the character has to do is dig.