Ruby and Sapphire belong to the Corundum gemstone crystal family, second hardest gemstone after Diamond. Ruby is very tough but it can still suffer damage if a cut stone, for even if the top flat surface, or face, of the gemstone is virtually unscratchable during everyday wear, still the side facets can be nicked with a hard knock at just the right angle.
However, when this type of opaque Ruby stone is cut cabochon, round with no facets, it is extremely tough, for not only is the material itself very hard, its cabochon cut simply deflects; it has no facets to nick.