Colored Sapphires Love Impurities

So Many Sapphires!

As a Ceramic Engineer, right up front I'm going to lay claim to sapphire. It's a ceramic. A single crystal ceramic, but ceramic never-the-less. For my engineering friends out there, we know it as alumina (Al2O3), and it's commonly referred to as corundum. Pure sapphire would be clear, kind of like diamond. The vibrant colors we see in sapphires (they can be all shades of blue, pink, yellow), are from a crystallography view, due to the presence of impurities. Let's look at the what & why with some nerdy gem stuff!

Blue Sapphires

It's weird to think that defects and impurities are responsible for the beautiful blues we typically think of for sapphires. The blue color comes from the substitution of titanium and iron atoms for aluminum in the crystal structure. Something new that I learned in researching this article is exactly how the blue color is produced, and if you guessedintervalence charge transfer—you're right!

Clear out the chemistry bunker cobwebs in your mind palace, and imagine you're an iron (Fe) atom. You're a transition metal, so you're OK with being either Fe2+ or Fe3+. Your neighbor titanium (Ti) however, is happier being Ti4+.

You both are being substituted into a spot in the crystal where Al3+ usually sits. If a Ti4+ atom goes into that location, it's going to create a charge imbalance, well, because 3 is less than 4. Being too positive is like to being too chipper, it's annoying.  

Your neighbor Fe2+ wants you to tone it down, and is willing to give you one of its electrons (making it more positive and you more negative). Then you can both be 3+ in your valance state, which matches the valance state of the original inhabitant, Al3+. Everyone wins!

Fe2+ + Ti4+ -→ Fe3+ + Ti3+

So now it gets a little wild. Playing nice with your neighbor and "sharing" an electron isn't the end of the story. Turns out you as Ti4+ aren't burly enough to wrestle the electron away from Fe2+ all by yourself.

Here come light to the rescue. White light is made of up different colors, and each color has its particular wavelength and associated energy. When light is shined on a sapphire, the yellow wavelength provides just enough energy for Fe to lob the electron over to you, and you to hold it in your orbit. Since the yellow light is absorbed in this process, it's subtracted out from the incident white light. This results in us (as humans now) seeing yellow's complementary color—blue—or the absence of yellow.  

 All this begs the question, what color is a blue sapphire in the dark?

For more details and diagrams click here.

Yellow Sapphires

Yellow is created in sapphires by the presence of just a few hundredths of one percent of iron (but not titanium). 

Pink Sapphires

Chromium inclusion causes the red color of rubies or the pink in pink sapphires. I find it interesting that pink is generally relegated to sapphire, while red corundum is firmly classified as ruby. The higher the concentration of chromium, the more intense the color red will be. At least 1% chromium must be present in corundum to get the deep red-ruby color.

Understand here that ruby and sapphire have the same crystal structure and parent material (both are corundum/alumina). It's the difference in impurities that account for the colors and name classification, which is typical for gemstones. (Aquamarine and morganite  are a similar example in the beryl family).  

Apparently whether or not something is a pale ruby or a pink sapphire can be quite the point of contention among gemologists. I prefer to think of them as happy pink sapphires, because really, who wants a sad, pale-pink ruby?

What's your favorite color of sapphire? Leave a comment below!