One hundred years ago, a ship sideswiped an iceberg on its way across the ocean, and the Titanic legend was born. Speaking of legend, James Cameron's film was so sweeping and dramatic that some folks think it must have been entirely fictional. But it was based on a true story, right down to the Heart of the Ocean.

Only, the real Heart of the Ocean wasn't a blue diamond. It wasn't heart-shaped. It wasn't ever owned by Louis XVI. And it wasn't called the Heart of the Ocean, although it's now known as The Love of the Sea. There's definitely a love story involved, though. Not to mention, geology.

This lovely and simple sapphire graced the neck of 19 year-old Kate Florence Phillips, a Worcester, England shop assistant eloping to America. Henry Samuel Morley, twenty years her senior and owner of one of the shops she worked in, had given her the necklace before they embarked on a new life together. They boarded as Mr. and Mrs. Marshall, with second-class tickets and likely first-class dreams. Henry had left his wife and child provided for. In San Francisco, he and Kate were to begin anew. One can imagine her face, glowing with happiness, with the necklace that symbolized their future glittering on her chest during dinners in the Titanic's elegant dining rooms.

Then came the iceberg, and the reality of too-few lifeboats, and women-and-children first as the band played and the passengers scrambled to survive. Henry died in the icy Atlantic waters. Kate made it off the ship and back safely to England on the Celtic. She carried nothing but a purse with her trunk keys, a pregancy, and her beloved sapphire necklace.

That's the human story of that deep-blue oblong. I, being a geology addict, am touched by their love story, but also by that beautiful necklace. What, when you come right down to it, is a sapphire? How did it form? And how did it end up nearly going down with arguably the most famous shipwreck in history?

It's possible the Love of the Sea isn't even a natural sapphire. The art and science of synthetic sapphires had emerged by 1902; ten years later, when Henry Morley went shopping for his new love and the Titanic sailed, over 7,000 pounds (3,200kg) of them were produced every year. But let's assume that Henry was a true romantic and a gentleman who would have preferred a naturally-occurring gem. It's more geological that way.

I suppose it isn't quite so romantic to point out that what he was purchasing was basically aluminum: aluminum oxide (Al 2 O 3 ). It's the crystalline form of aluminum oxide, which makes all the difference. This crystalline aluminum mineral is called corundum, and people have been wearing it with pride for thousands of years. It's also a bonza industrial abrasive, and clear slices of synthetic corundum are used to make bullet-proof "glass." Corundum is, in fact, the 2nd hardest mineral: a 9 on the Mohs hardness scale (if I ever open a restaurant, I'll call it Mohs Diner). So Kate was wearing two minerals used on the hardness scale: diamond's number 10. And you now have a new bit of cocktail party conversation. Accessorize accordingly.

In its pure form, corundum is pretty much clear. Like so many things, impurities are what makes it fascinating. Ruby, the deep-red variety, gets its color from a trace of chromium. Padparadscha, a rather charming pink-orange gem, contains chromium, iron and vanadium. And sapphire, our gem of interest, can occur in a variety of colors from yellow to purple to true sapphire blue. The blue color is a result of traces of titanium and iron. But it's not just down to trace elements: it's about chemistry, too. That ocean blue is caused by a little thing called intervalence charge transfer. I wish I could translate that into plain English for you, but I have not yet studied enough chemistry to manage it. It's got something to do with electrons. And it means that, while 1% chromium is required to make a ruby ruby-red, it only takes .01% titanium and iron to make a sapphire sapphire-blue. Neat, eh?

For an even better blue, sapphires can be heat-treated. Even the Romans did it. But the final oh in that deep, dark blue is OH, hydroxide. Sapphires with higher OH intensities have an almost TARDIS-blue hue. Less OH means a paler sapphire. My very own wee Montana sapphire, for instance, is probably low in OH, although I don't consider it low in ooooh. Yes, I'm partial.

That was the easy stuff. "Here's where it gets complicated," as Amy Pond says. Because it's not enough for me to know chemical compositions and why sapphires are blue (or purple, or yellow, or clear). No, I had to go and wonder, "How do these form, then?" I thought it would be a bit of a lark. You know, read a couple of papers about pretty things, breeze right through 'em and be able to tell you precisely how sapphires begin and grow. By the end, a passing acquaintance with the geological dictionary had blossomed into an intimate friendship as I cried on its shoulder, and I'd decided a petrology course is most certainly in my future. Yeah. It gets that complicated. What follows is a super-simplified version.

The basic requirements for sapphire formation appear to be magma and country rocks rich in aluminum but poor in silica. You'll find sapphires in some volcanic fields, and in metamorphic rocks like gneiss, mica schist and sometimes marble. Picture magma, that hot melty stuff. We begin with ultramafic and mafic melts, which is basically a fancy way of saying magmas that have very little silica and a lot of magnesium and iron. This hot rock rises, on account of being hot. As it ascends from within or near the Earth's mantle, it's going to encounter the country rocks - the locals, who were already chilling, probably lived there a while. Some of those country rocks have a low silica content, but lots of aluminum. As they get invaded by the hot stuff, they melt a bit themselves and mix in.

Now, different minerals start to crystallize out while magma's underground. It's a process called fractionation. It's a fascinating process, and one we'll explore in-depth someday. For now, it's enough to know that some minerals crystallize out before others. Some of these minerals may very well be our very own corundum, with its trace elements. The whole process of creating sapphires is, judging from the abundance of "most likelies" and "mays" in these papers, still rather fuzzily understood. But we do know we find sapphires in basaltic lava fields or eroded remnants thereof, mafic dikes, places where granitic pegmatites interacted with silica-poor country rock, rocks formed by contact metamorphism (basically, where magma baked the country rock into something a little different), and in other metamorphic rocks. So we know it takes high aluminum content, low silica, lots of heat, mixing, and cooling to cook up a sapphire. Pretty much. And then they might be carried up to the surface by an eruption, or left quietly in place to erode out later.

Sapphires are buggers to extract from their native rocks, but they're tough enough both physically and chemically to survive erosion, and so can be economically mined from sediments, especially stream deposits. They're left behind when the lavas they grew in are weathered down to soil, where they can be recovered. And then the gem-quality ones are cut, polished, and end up as jewels that might, just possibly, get rescued along with their owner from a famous sinking ship. Lovely!

Image Credits:

The Love of the Sea photograph reproduced with the kind permission of John White of the Nomadic Preservation Society.

All other images filched from Wikimedia Commons.

References:

Beran, A. and Rossman, George R. (2003): OH in naturally occurring corundum. European Journal of Mineralogy, v.18,N4, pp.441-446.

Coenraads, R., Sutherland, F. and Kinny, P. (1990): The origin of sapphires: U-Pb dating of zircon inclusions sheds new light. Mineral. Mag., 59, 465-79.

Srithai, B. and Rankin, A. (2006 ): Geochemistry and genetic significance of melt inclusions in corundum from the Bo Ploi sapphire deposits, Thailand. In Pei Ni and Zhaolin Li (Eds), Asia Current Research on Fluid Inclusion (ACROFI-I).

Charles Pellegrino: Ellen Mary Walker. Charles Pellegrino Website. Retrieved 4/14/2012.

Nomadic Preservation Society: Inspiring Jewellery Goes On Show… Retrieved 4/14/2012.

Want moar geology? Check out David Bressan's post on the iceberg that sank the Titanic.