How to test rubies with black light

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 lustre. Corundum may be many colours, but only the red variety is a ruby. Rubies often form in aluminium-rich metamorphic rocks or marbles and get their colour when chromium replaces some of the aluminium 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 colour emitted from the samples, if any. A sample that does not glow is not fluorescent.

Record the colour 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 jeweller to examine the stone. Some rubies do not fluoresce if the iron content is too high.


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

Things You'll Need

  • Black light
  • Ruby samples
  • Dark room
  • Notebook
  • Pencil
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

Tracy Barnhart is an earth science expert. A professional geologist with over 16 years of technical writing experience, she has expanded her writing skills to include instructional articles on business, parenting, finance and science. She has Bachelor of Science and Master of Science degrees in geology from Furman University and the University of South Carolina.