Hope Diamond - the science of diamonds


	...the science of diamonds

From an interview with Jeffrey Post, Curator of the National Gem and Mineral Collection of the Smithsonian Institution:

"People are so excited about this three-hundred-fifty-year human history of the Hope Diamond, and what I'd like to try and remind them is that, in fact, the real history of the Hope Diamond began many, many years before that – perhaps as many as three billion years before it was ever seen by human eyes.

Crystal clear diamonds are made of carbon, forged under tremendous pressure and heat – about 1,200 degrees centigrade – about one hundred miles below the earth's surface in the upper mantle. Under these conditions, carbon atoms come together and link to form the structure of the mineral we know as "diamond." Over time, these carbon atoms can link together to build crystals that eventually become large enough for us to see, and this may happen over a period of one to three billion years. Age-dating indicates that many diamonds are as old as three billion years, but most of them are at least a billion years old."

The next step is that these diamonds are brought to the surface. And this happens in a very unusual kind of volcanic eruption. The molten rock or magma that forms in the upper mantle literally carries these diamonds – like passengers up through a conduit pipe – to the surface of the earth at great speeds, probably thirty or forty miles per hour. It may only take a few hours fir the molten rock to reach the surface, where it erupts explosively, and the diamonds are entombed in the rock as this magma cools, forming a special kind of volcanic rock called kimberlite. The kimberlite erodes over time, leaving behind a more resistant rock, the very hard diamonds, which accumulate in gravel deposits.

This is the kind of deposit that was formed in India. The Hope Diamond came from India, probably from the Golconda area, which was one of the active diamond mining areas in the world at that time, and perhaps from a mine known as the Kollar mine which was well known for producing colored diamonds. We know that the deposits there erupted probably about a billion years ago, so the Hope Diamond would have been brought to the surface at least that long ago.

There are very few blue diamonds in the world, and as far as we know, the Hope Diamond may be the largest blue diamond. Why is it blue?

Well, it turns out that when this diamond was forming deep within the earth, in addition to the carbon atoms that were locking into place to build the diamond structure, there were a few impurities of boron atoms that happen to substitute for some of these carbon atoms. And it's the light interacting with the boron atoms in the diamond structure that results in the blue color. Very few diamonds have this boron impurity, and therefore there are very few blue diamonds.

Now this impurity also gives the diamond some special properties. First of all, the Hope Diamond is known as a Type II-B diamond, a classification used to describe diamonds with certain properties. And one of the most intriguing properties of a Type II-B diamond is that it's semi-conducting; it can conduct an electric current, whereas other diamonds can't do that.

The Hope Diamond also has a very unusual phosphorescence. That is, when you take the Hope Diamond into a dark room and expose it to ultraviolet light, then turn off the ultraviolet light, the diamond will glow a deep ember-orange color. A very few other blue diamonds have been known to show the same orangey phosphorescence, although none that I'm aware of to the same intensity as the Hope Diamond. And so, in some ways, that's become a signature of the Hope Diamond, an intriguing and almost mystical property that has added to the wonder of this important blue diamond."

It may also have contributed to the rumors of the stone's supernatural powers...