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.




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.

Crystal structure of corundum. Image credit: NIMSoffice at en.wikipedia.

I suppose it isn't quite so romantic to point out that what he was purchasing was basically aluminum: aluminum oxide (Al2O3). 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.





Sapphire from Madagascar. Image credit: Wikimedia Commons.

Sapphire from Madagascar. Image credit: Wikimedia Commons.

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!

Sapphire crystals. Image credit: Wikimedia Commons.

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All other images filched from Wikimedia Commons.







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