Sapphires are one of the most  precious gemstones out there.

They come in a rainbow of colors,  they’re super hard, and they can stand up to daily wear as jewelry. They’re  even durable enough that we use them in industrial applications.

Plus, we can find them right  here near our studios in Montana! Which is one reason why they’re  the first item in our new SciShow Rocks Box subscription - stick  around to the end to learn more about that. But the really neat thing about Montana  sapphires is that there’s a big mystery surrounding these pretty little stones.

Even  though we know where to find them, researchers have no clue where  most of them originally came from. [♪ Intro] So to start out, there are two basic kinds of  mineral deposits: Primary, and alluvial. Primary is what you picture when you  think of a miner with a pickaxe and TNT, blowing holes in the sides of mountains and bringing up carts full of  precious minerals or ores. It basically just means  that the minerals are in the same spot where they formed, more or less.

Meanwhile, mining from an alluvial  deposit of minerals is more like a prospector panning for gold in a  river, just on a way, way bigger scale. Alluvial deposits are formed when  your fancy desirable mineral is washed downstream by a river, and  collects in one location. They can also be referred to as placer deposits  or placer deposits , since they were placed there.

These alluvial sapphires are  found in a few places in Montana, including Rock Creek, Dry Cottonwood  Creek, and the Missouri River deposits. But we don’t know for sure where they eroded from. And while you’d think this  would just be a matter of tracing the rivers upstream, you’d be wrong.

See, researchers are pretty  sure that these deposits aren’t even the first time these  sapphires have been moved by erosion. It’s likely that they’ve been  knocked around the landscape for upwards of 2 million years. So while the investigation is ongoing,  there’s definitely a possibility that the original source vein for these Montana  sapphires has completely washed away.

However, not all of Montana’s  sapphires are alluvial. There’s one place called Yogo Gulch  that has primary deposits of sapphire, which are aptly called Yogo sapphires. One of the very first stones  I ever bought for my wife. This cache dates to more than a hundred years ago when a prospector was  searching for gold in the area.

He didn’t manage to get all that  much, but he did find a bunch of odd light blue stones, which  turned out to be sapphires. Mining began in the area at the  tail end of the 19th century, but petered off in the early 20th century. See, while Yogo Gulch sapphires are pretty, they’re not often great as  jewelry-grade gemstones.

Most tend to be smaller than a carat,  so for most of the mine’s operation, the sapphires they sold went towards  watchmaking rather than jewelry. Because of both some strings of bad  luck and encroaching competition from synthetic sapphires, Yogo Gulch ended up ceasing mining operations in the late 1920s. Over the ensuing decades, the mine has  traded hands and restarted production a few times, most recently in 2021, which is exciting news for fans  of these particular stones.

But what makes these sapphires so special? Sapphires are a type of corundum,  which is made of aluminum and oxygen. The color of the stone is determined  by other things in the stone, like gallium, iron, titanium, and other things.

Rubies are also corundum's, so they’re  basically identical to sapphires chemically - we just call them a different name …  because. Honestly, it’s mostly marketing. Corundum's can form in two ways -  either directly from the hardening of melted rock underground, or  through the later transformations of those hardened materials.

Research supports the idea that  the alluvial Montana sapphires took the latter route, while the  Yogo sapphires are more likely formed right from the hardening rock, though  there’s still some debate there, too. However they formed, there’s a quirk  of their geochemistry that makes these stones special: Yogo sapphires  have traces of titanium and magnesium. And while the alluvial sapphires are  similar in some ways to those found in Colombia and Myanmar, they’re  still distinct in the ratio of iron, magnesium, and gallium.

Which gives them a unique array of colors  found in few other places in the world. And if you sign up for our new  SciShow Rocks Box subscription, you’ll get one of each of these Montana sapphires! The Rocks Box delivery will  include one Yogo sapphire, one sapphire from the Missouri River, and a tumbled sapphire from either  the Missouri River or Rock Creek.

And then each month, a new rock or fossil will arrive in your mailbox. And those of you who subscribe  early will be members of the SciShow Rocks Box Founder's Club, with  exclusive access to Founder's Club merch. To get started on your SciShow  mineral collection like me, visit scishow.rocks for all the details! [♪ OUTRO]