According to legend, Helen of Troy was so beautiful that her face launched a thousand ships, leading to the Trojan War  more than 3000 years ago.

But other stories suggest that  it might not have been her face, but her jewelry that was  the key to her desirability. She was said to possess a rare and magical stone that shone like a star.

While this might seem like a tall tale that got distorted over the millennia, the existence of starry gemstones is no myth. Star sapphires and star rubies  really do reflect six pointed stars. And they owe that trick not to their purity, but to some tiny imperfections  in an otherwise common mineral. [Intro Music] As gemstones go, Corundum is pretty easy to find.

It’s made from aluminum and oxygen  that combines to make hexagonal or double-pyramid-shaped crystals,  and it exists all over the planet. But the thing about this aluminum oxide structure is that it’s incredible hard. In fact corundum is the second  hardest natural substance we know of, second only to diamond.

It crystallizes from slow-cooling  igneous and metamorphic rocks, in places that are rich in aluminum, but have no silica. And because it’s so hard, it readily survives erosion and weathering. In fact, back in 2023, we made a video about how  corundums are found in alluvial or erosion-formed deposits here in Montana, and how nobody is really sure where  they came from in the first place.

In its natural form, corundum  is clear and colorless, but this isn’t how the mineral is usually found. Instead, it occurs with tiny impurities  inside its crystal structure, which have the ability to drastically  change its appearance and value. Most commonly, the crystals are  intimately mixed with iron oxide minerals like hematite and magnetite, to create a dark-colored mass known as emery.

In this form, the corundum  is no good as a gemstone, but when it’s crushed into a powder  it creates an incredibly hard grit that can be used as an abrasive. This grit, graded to different sizes, is used on grinding wheels and emery papers to grind and polish glass and  metals to a very fine finish. Even if you’re not into industrial finishes, you’re likely to have emery in your home as part of the emery boards used to file nails.

But while corundum in that form helps  to keep your hands looking neat, other variants can elevate  your look to a whole new level. The precious gemstones ruby and  sapphire are also forms of corundum, just with different kinds  of impurities within them. Rubies, for instance, get  their red colour from chromium.

Just one in 100 aluminium atoms in the  crystal structure is replaced by this element, but it’s enough to transform a simple hard rock into something much more special. And sapphires have an even  lighter touch of impurities. In these, just one in 10,000 aluminium  atoms is replaced by titanium and iron, which is enough to turn the  crystal a range of blues.

Other combinations of contaminating metals can create a range of colors in corundum, including yellow, green, purple and brown. All of these varieties tend to  be highly valued as gemstones, not just for their color, but also  for their hardness and resilience. Which is a great property for a stone to have if it’s going to be bashed  about as jewelry all day.

But there’s one kind of impurity that can make your gemstone a little extra…extra. Under the right conditions, rubies and sapphires can reflect a single six-pointed  star when they catch the light. The phenomenon is known as asterism, meaning ‘a marking with stars’, and gems that show it are  given the imaginative names: star rubies and star sapphires.

It’s this kind of star sapphire that was rumored  to be the source of Helen of Troy’s allure. The starry reflections only show  up if your gemstone is polished into a smooth, rounded stone called a cabochon. Which isn’t the easiest thing to do because, remember, these corundums are super hard.

But doing so reveals a whitish star reflection that drifts across the gem’s  surface as you move it in the light. Unlike color, which is caused by  substitutions in corundum’s structure, asterism is caused by inclusions of another  mineral, trapped inside the crystal. In the case of the star corundums, these inclusions are extremely  fine needles of rutile, made of titanium oxide, that are around 200 microns  long and just 2 microns wide.

That’s as much as 50 times  thinner than a human hair. Because it’s so fine, these inclusions  are sometimes called rutile silk. The rutile needles aren’t scattered  randomly inside the crystal though.

Instead, they’re grow in line  with the corundum crystal itself, oriented parallel to its hexagonal crystal faces. The result is an interlocking network of  needles at 120 degrees from each other. The needles reflect the  light entering the gemstone, just like the shiny surface of silk thread, and the reflections from each of  the needle orientations combine to make a six-armed star.

Because of their seemingly magical reflections, star rubies and star sapphires are  highly prized in the gem world. If the legends are anything to be believed, that has been the case since  the Trojan War, 3000 years ago. And starry corundums are still making the news.

The largest known example was certified in 2016, nicknamed the Star of Adam. It’s a star sapphire weighing  1400 carats, or nearly 300 grams, and big enough to fill the palm of your hand. Now squirrelled away in a private collection, it’s estimated to be worth at least $100 million.

And another giant star sapphire  called the Star of India, weighing in at more than 500 carats, was desirable enough to draw the  attention of thieves in 1964. It had been displayed at the  American Museum of Natural History, when it, along with a handful  of other notable gems, were stolen and removed from the  premises via a bathroom window. Although the thieves themselves  were caught within a few days, it took until the following year  before the Star of India was found, in a bus locker in Florida!

From legendary wars to modern-day crimes, it’s incredible to think that all of this buzz is really about the impurities  inside humdrum corundum, rather than the mineral itself. Which just goes to show that if  you want to shine like a star, it really is the imperfections that count. And SciShow Rocks Box subscribers have a chance to showcase their  own imperfections this month.

We’re sending each of them a large  corundum from the Mysore region of India. Some of the stones will even have the  asterism that Helen of Troy’s jewelry may have been rocking 3,000 years ago. if you'd like to become a subscriber just click the link in the description below [ OUTRO ]