Questions about Crystals

July 5, 2008 11:03 AM

How do we know what makes up a gemstone? And did people know in the previous centuries when (I assume) these techniques weren't around

I just finished reading The Blue Carbuncle, and in it Holmes says something to the effect of "all this fuss over a small piece of crystallized charcoal." I know that rubies are just doped sapphires, which is just crystal aluminum oxide, so my impression is that the boundary between the two might not be hard and set, yet there are all these gem stones and tons of different names and chemical compositions, so how do/did we determine what something was? How did Holmes know that diamond was carbon (although apparently he was wrong about the carbuncle, which is what got me wondering)?

Also, is there a good natural history book about all this?

posted by Large Marge to science & nature (6 comments total)

I'm not sure these are natural history books but...

Gems, Their sources, Descriptions and Identification by R Webster, revised by B Anderson. Published by Butterworths, 1962.
Includes history and lore, chemical and physical properties, occurrences. Out of print.
isbn 0208014918, 0408206535 & 0408011483

Gemstones of the World by Walter Schumann is also good, available new on Amazon.
posted by Fins at 12:04 PM on July 5, 2008

It was wide spread knowledge by that point in the 19th century that diamond was an alternate form of carbon.

The guy who developed a way of producing calcium carbide cheaply in quantity, and thus acetylene, was attempting to come up with a way of creating diamonds out of carbon.

How did they find out? It's actually pretty easy. If you take a diamond and heat it in a non-oxygen atmosphere, it changes into graphite. (If you do it in an oxygen atmosphere, it burns.)

By about 1890, chemistry as a science was quite advanced. The first aniline dye (mauve) was developed in 1856, and by the end of the century dye-making was very advanced. Emission spectroscopy also was developed in the 1850's, which made elemental analysis possible of anything you could figure out how to vaporize.

As to Holmes getting it wrong, I would point to a different story, in which Watson tried to tell Holmes about the heliocentric theory. Holmes response was something to the effect, "Why would I care whether the earth goes around the sun or vice versa? Now that you've told me I shall try to forget it. I believe our brains can only hold so much, and anything useless takes room away from things I might need." (I think that was in "Study in Scarlet" but I could be wrong.)

As a detective, Holmes needed to know that a carbuncle was rare and valuable, but he certainly did not need to know what it was made of chemically.
posted by Class Goat at 12:33 PM on July 5, 2008

In ancient times, these categories for gemstones were not as rigorous as they are now. An example would be jade. Both in Asia and in Mesoamerica, there were various green, green/blue, and bluish stones that were considered "jade," although now we only allow that distinction with two different stones (jadeite and nephrite).
posted by anansi at 1:00 PM on July 5, 2008

Carbuncle's blog definitions
posted by hortense at 4:34 PM on July 5, 2008

I know that rubies are just doped sapphires, which is just crystal aluminum oxide, so my impression is that the boundary between the two might not be hard and set, yet there are all these gem stones and tons of different names and chemical compositions, so how do/did we determine what something was?

Stones were once classified on color + beauty rather than chemical/structural composition. Yes, sapphire and ruby are the same stone...but the "doping" is by nature in natural stones depending upon the heat to which they were exposed. Citrine and amethyst are likewise the same stone 'doped' by varying amounts of heat (and of course are both just forms of quartz, like clear quartz, rose quartz, etc.)

Seconding Gemstones of the World as the basic reference on the classification of stones and which gems fall into which categories. It won't answer all of your questions, maybe, but read through it for the basic gist of how this works -- it'll likely inspire more questions.
posted by desuetude at 11:21 PM on July 5, 2008

This is probably a minor derail, but Sherlock Holmes was an accomplished amateur chemist, always tinkering at his table by the fire and publishing his observations in esoteric journals. He would certainly have been aware of the different allotropic forms of carbon.
posted by ikkyu2 at 11:43 PM on July 5, 2008