How can you identify a genuine ruby?

Genuine rubies are usually recognised by their kind of silky inclusions, rutile needles, plus a bright red fluorescence when you shine long-wave UV light, and also by a high refractive index that sits around 1.762 to 1.770.

If you look through a jeweller’s loupe, a natural ruby will commonly show intersecting needle-like features or other crystal inclusions. Synthetic flame-fusion rubies, however, often present curved growth striae, along with tiny gas bubbles. A trained gemmologist will also verify pleochroism, since natural corundum shows varying hues of red and purplish red depending on the viewing direction.