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How to Test Rubies With Black Light

Rubies are one of the few fluorescent minerals.
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Things You'll Need:

  • Black light
  • Ruby samples
  • Dark room
  • Notebook
  • Pencil


You can perform this test on other fluorescent minerals such as fluorite and calcite.

Rubies are the red variety of the mineral corundum. Rough ruby crystals form in hexagonal prisms and plates. An aluminum oxide, ruby is the second-hardest gemstone — it's nine on the Mohs hardness scale — and has a nonmetallic luster. Corundum may be many colors, but only the red variety is a ruby. Rubies often form in aluminum-rich metamorphic rocks or marbles and get their color when chromium replaces some of the aluminum in the crystal lattice. Rubies are one of the few fluorescent minerals. You can see the fluorescence under a black light.

Gather your ruby samples and a black light then close yourself in a dark room. Shut all the doors and cover the windows to make the room as dark as possible.

Turn on the black light, shine it directly onto each of your ruby samples and observe the color emitted from the samples, if any. A sample that does not glow is not fluorescent.

Record the color of the fluorescent light emitted from each sample in a notebook. Rubies emit a fluorescent red light. If your gemstone does not fluoresce and you think it is a ruby, ask a reputable jeweler to examine the stone. Some rubies do not fluoresce if the iron content is too high.

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