There's the scarlet thread of murder running through the colorless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it.

Strange indeed is human nature. Here were these men, to whom murder was familiar, who again and again had struck down the father of the family, some man against whom they had no personal feeling, without one thought of compunction or of compassion for his weeping wife or helpless children, and yet the tender or pathetic in music could move them to tears.

A study in scarlet, eh? Why shouldn't we use a little art jargon? There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it.

The man might have died in a fit; but then the jewels are missing," mused the Inspector, "Ha! I have a theory. These flashes come upon me at times... What do you think of this, Holmes? Sholto was, on his own confession, with his brother last night. The brother died in a fit, on which Sholto walked off the treasure! How's that?"
"On which the dead man very considerately got up and locked the door on the inside," said Holmes.

Can a husband ever carry about a secret all his life and a woman who loves him have no suspicion of it? I knew it by his refusal to talk about some episodes in his American life. I knew it by certain precautions he took. I knew it by certain words he let fall. I knew it by the way he looked at unexpected strangers.

Holmes took up the stone and held it against the light. "It's a bonny thing," said he. "Just see how it glints and sparkles. Of course it is a nucleus and focus of crime. Every good stone is. They are the devil's pet baits. In the larger and older jewels every facet may stand for a bloody deed. This stone is not yet twenty years old. It was found in the banks of the Amoy River in soutern China and is remarkable in having every characteristic of the carbuncle, save that it is blue in shade instead of ruby red. In spite of its youth, it has already a sinister history. There have been two murders, a vitriol-throwing, a suicide, and several robberies brought about for the sake of this forty-grain weight of crystallised charcoal. Who would think that so pretty a toy would be a purveyor to the gallows and the prison?