• Question: What makes rubies red and emeralds green?

    Asked by grace2112 to Sinead, Jessamyn, Eleanor, Chris, Adam on 18 Nov 2013.
    • Photo: Jessamyn Fairfield

      Jessamyn Fairfield answered on 18 Nov 2013:

      Great question! In both rubies and emeralds, the color of the gem actually comes from impurities in the base mineral. So for rubies, most of the stone is a form of aluminum oxide, but the color is due to chromium impurities that might be less than 1% of the stone. We can treat a ruby to remove impurities and see the color go away, or alternately it’s possible to actually introduce additional chromium to the stone to enhance its color.

      What the impurities do is absorb specific colors of light, so in rubies the chromium atoms are only letting red light pass through the stone. Chromium impurities are also the color source in emeralds, but the base mineral is different which affects what colors of light the chromium atoms absorb.

    • Photo: Adam Murphy

      Adam Murphy answered on 18 Nov 2013:

      Hey Grace, really cool question. Gemstones are something that really interest me.

      Jessamyn’s completely right in her answer! Rubies are a kind of crystal called corundum (which is one of my favourite words) which is made of aluminium oxide (which is like rust, but for tin cans instead of iron). If there’s chromium in it, it absorbs some light in the blue area of the rainbow, leaving red behind.

      A ruby was the source of the red light in one of the very first lasers!

      If corundum has other elements in it, it can be a load of different colours, like blue or yellow. Then it’s called a sapphire.

      Emeralds are made if there’s chromium in a different type of crystal called beryl, which has silica, aluminium and beryllium in it. You can actually have red emeralds if instead of chromium, you have a different element called manganese.

      Also everyone has a gemstone associated with their birth month. Mine’s in January, so it’s Garnet, which is like a ruby. What’s yours?

    • Photo: Eleanor Holmes

      Eleanor Holmes answered on 18 Nov 2013:

      Hi Grace,

      I didn’t know all of this about gemstones and I found reading Jessamyn and Adam’s answers extremely interesting.

      It’s so cool how “impurities” in a substance can give it some of it’s most interesting properties. It’s the same with semiconductors (which I work with a lot). We work so hard to get absolutely pure, atomically-perfect crystalline semiconductors. Then we have to “dope” them with impurities so that they will conduct electricity in the way we want. The interesting effects come from the impurities!

      Oooh, deep! If you look at it the right way, physics is full of life metaphors.

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