Their faces are rough and deeply grooved. Transparent to translucent, they sometimes appear opaque; colourless, greyish, bluish, deep blue (sapphire), red (ruby), yellow (yellow sapphire), green (green sapphire).
Rubies are pleochroic and may fluoresce, which accentuates the intensity of their red (pigeon’s blood) colour. The cabochons sometimes have a six-branched asterism.
They are found in metamorphic shales, marbles and alluvial deposits are derived from these rocks.
The ruby owes its colour to traces of chromium oxide in its crystalline network, the sapphire to traces of iron and titanium oxides. The asterism that characterizes certain stones and which manifests itself by the appearance of a luminous six-pointed star is due to the presence of fine inclusions of rutile needles arranged in the crystalline network.
These gems are exploited in the alluvium resulting from the disintegration of the metamorphic rocks that contained them.