Published: 20 December, 2006

Real ruby is considerably harder, more dense than 'fake' gemstone

How can you tell if a ruby is real or fake? Is there a test I can do?

Ruby, the gemstone, is the red variety of the mineral corundum. Corundum is crystalline aluminum oxide (alumina). Ruby's red color comes from a small amount (a percent or so) of chromium impurity that substitutes for aluminum in the crystal structure. Other impurities give corundum other colors: gem sapphire occurs in a blue variety where the impurities are iron and titanium Naturally occurring ruby crystals formed slowly at high temperature deep in the earth. A rock containing ruby is unmistakably "natural" as the rock texture is not reproducible in the lab. Also diagnostic are small mineral inclusions in natural ruby.

"Fake" ruby results from mis-identification of another red mineral, commonly garnet or tourmaline, or of colored silica glass. These mis-identifications are relatively easy to detect making use of physical properties. For instance, ruby is much harder than tourmaline, garnet, or especially glass. As well, fake rubies are often less dense than the real thing. It's easy to do these physical tests.

Now comes the hard part: telling natural from synthetic ruby As is the case for diamond, a lot of ruby crystals have been grown (synthesized) in the lab. Growing rubies is by no means easy, but unlike mis-identified ruby, synthetic ruby is no less "real" than natural ruby. It can be extraordinarily difficult to tell the two apart. In the Verneuil synthesis process (used for over a century), rubies are grown by dripping fused alumina onto a plate, where it crystallizes. Characteristically curved growth features--observed with a microscope-- remain in the synthetic single crystal and are tell-tale signs of a synthetic origin. Heat treatment changes the internal features of the crystal making detection of this type of synthetic ruby more difficult. More recently, the appearance and physical properties of ruby crystals grown in a flux can be made to be essentially identical to those of natural ruby. Chemical analysis to detect trace amounts of impurities characteristic of the flux--like lead-- may be the only way to tell natural from synthetic.

Interested is seeing some natural ruby and corundum? Come to the Snee Hall mineral museum on the Cornell campus!