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Rubies? Walks like a duck…..

April 6, 2020









Here’s a puzzle to keep you occupied.


  • It looks like a duck.
  • It quacks like duck.
  • It walks like a duck.

But when you do an X-Ray, it has the skeleton of a snake.

ruby maybe

Ruby (?) under plane light

UV ruby

Synthetic ruby (?) fluorescing under UV light

We received a magnificent donation of more than 400 gemstones (more on that later), all faceted. As part of the curation process, these need to be identified, or verified. This specimen was marked as a ruby. It’s red (looks like a duck) and it fluoresces in ultraviolet light (quacks like a duck). But the skeleton…….

The National Science Foundation funded an excellent infrared spectrometer for my lab, and one of the most useful attachments is Attenuated Total Refectance (ATR). The sample is held against a diamond, and the infrared beam bounces off. Anything you can put against that diamond- powder, lab alcohol, gemstones, anything except sulfuric acid- you can get a spectrum on. There’s a device to hold the sample down, and it’s even a torque wrench so you can’t tighten it too much. It’s robust enough that I use it to teach middle schoolers about spectroscopy.

ATR is great for mineral identification. It gives information about the basic crystalline framework for each different mineral. A ruby is the mineral corundum, aluminum oxide, of Al2O3. Here’s a corundum reference from RRUFF, which is a public database for mineral data.

Created with GIMP

Here’s how you read one of these. The graph shows areas where infrared radiation is absorbed. If there’s nothing there, 100% gets through. The “peaks” in this case hang down like stalagtites from the top of the graph. Units on the X-axis are reciprocal centimeters (cm-1), the number of waves per centimeter. If you want to think about it as frequency, low frequency is to the right and higher frequency is to the left. Error in the x is ±4 cm-1.

Corundum unks

But now let’s compare our unknowns with reference material. The RRUFF sample has a major absorbance at 553 cm-1, with a small shoulder peak at 634 cm-1. The reference from the Museum Collection, NCSM 4840, probably needs to be revisited. It has only an absorbance at 628 cm-1. Perhaps it is a synthetic mineral of some sort. The Ruby (?) unknown is our strange duck, and it, too, lacks the 553 cm-1 absorbance. So do the other two unknowns from this collection, a yellow sapphire and an “alexandrite” sapphire.

Next look at the RRUFF reference mineral. The peaks are nice and sharp. This comes from a lot of aluminum-oxygen bonds lined in exact crystalline marching order. The %transmittance on the y-axis is also about 40%. This is a very strong absorber. In all of our unknowns, the peaks are much weaker, and broader. This means that they are not as crystalline as the RRUFF reference, and it’s possible that we are looking at the coloring agent, scattered throughout the gemstone. The gemstones are less crystalline, which is common in synthetic minerals.

So, our red duck of a gemstone is not a corundum. It is also not:

  • Spinel
  • Garnet
  • Cuprite
  • Glass
  • Tourmaline (rubellite)
  • Red beryl

If you have ideas, or just want to fool around with the data, you can find reference materials for ATR at RRUFF to test your hypotheses. Download the spectrum, but when it come time to save it, use .csv instead of .txt. Then open it in Excel or another spreadsheet program and make a graph of the data. You may need to change to format on the number.

Good luck!



2 Comments leave one →
  1. June 14, 2020 5:11 pm

    Your story about rubies was very helpful to me in ways but you see I have what I believe to be a large un cut ruby I found it is 1,333.95 carets it weighs 8ounces I found it here in North carolina I want to find out if it’s really a ruby I don’t know who to go to for help can you help me. Please

    • June 15, 2020 1:09 pm

      There’s a few things to try. First, hardness. Rubies are red corundum. Will it scratch a piece of quartz? If it does, you have corundum. Second, crystal habit. Corundum has a hexagonal crystal habit. Look at the outline for hexagonal shapes and the 120 degree angles you’d see on a stop sign. Third, try a UV light. Rubies will fluoresce under the longwave ultraviolet. If you are anywhere near Franklin, NC, there’s the museum staff, rock shop owners and gem mine owners who will help you. Two other places to look for help: your local gem and mineral club and on A list of North Carolina clubs can be found at the Southeast Federation of Mineralogical Societies at Some time in the future, our Museum will re-open, and some time after that, the Naturalist Center will re-open and be available to assist with identification.

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