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The Blue Carbuncle (1892)

The Sherlock Holmes detective stories are famous. They were written in Victorian times by Arthur Conan Doyle. Although the stories are supposed to be about Homes' logic and reasoning capability, the stories incorporate many strange and exotic elements. Here are two passages from one of the short stories, "The Adventure of the Blue Carbuncle" from The Adventures of Sherlock Holmes

Sapphire
Sapphire



Ruby
Ruby
He held out his hand and displayed upon the centre of the palm a brilliantly scintillating blue stone, rather smaller than a bean in size, but of such purity and radiance that it twinkled like an electric point in the dark hollow of his hand.
Sherlock Holmes sat up with a whistle. "By Jove, Peterson!" said he, "this is treasure trove indeed. I suppose you know what you have got?"
"A diamond, sir? A precious stone. It cuts into glass as though it were putty."
"It's more than a precious stone. It is the precious stone."
"Not the Countess of Morcar's blue carbuncle!" I ejaculated.
"Precisely so."
         . . . . . . . . . .

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 southem 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 crystallized charcoal."
Diamond
Diamond



Garnet
Garnet



This is a vivid passage, but unfortunately it is nonsense. Peterson has found that "It cuts into glass as though it were putty", and so identifies it correctly as a precious stone (particularly since it has a good colour and purity). Holmes says that the stone is the famous Blue Carbuncle. The word 'carbuncle' is not used nowadays in its original meaning as a gemstone, although it has been used in its alternative meaning as a blemish. It used to mean a red stone, such as a garnet. Garnets can be many colours but they cannot be bright blue. Also garnets, while being precious stones, are hardly in the same league as rubies, and this is obviously an extremely valuable stone, "the precious stone" indeed! So it cannot be a garnet.

Rubies are sometimes called carbuncles. They are red corundum, the hardest stone there is apart from diamond, and they would certainly cut glass. A good ruby is indeed extremely valuable. Rubies can be described as carbuncles. But it seems strange to talk of a "ruby red" ruby - why not just call it a ruby rather than a carbuncle? What is more, there is nothing "remarkable" about this stone being a blue version of a ruby, since that is what a sapphire is! In fact, rubies are more valuable than sapphires. Finally rubies are sapphires are made of aluminium and oxygen, rather than "crystallized charcoal".

Sherlock Holmes doesn't correct Peterson who calls the stone a diamond. He admires the "glints and sparkles", describes it as extremely valuable, and finally calls it "crystallized charcoal". Diamond are made solely of carbon, the same as charcoal,although a different crystalline structure. Diamonds also sparkle with refracted light, and they can be many different colours. However diamonds are usually thought of as transparent, not red, and they have never been called carbuncles.

This all sounds as if Arthur Conan Doyle thought there was a precious stone called a carbuncle, which was extremely valuable and usually red, although this one was bright blue, which was "remarkable" and made it even more valuable, and it was made of carbon. However, it wasn't a garnet, ruby, sapphire or diamond. All in all, it sounds as it he did not know much about about precious stones!

However, the weight sounds reasonable. Forty grains are just under 14 carats. Presumably, Arthur Conan Doyle learned about grains while training as a doctor.