A wife of noble character who can find?
She is worth far more than rubies.
PRV 31:10 (NIV)
A ruby is a first order gemstone formed from red corundum (aluminium oxide) tinted by traces of chromium - any colour of corundum other than deep red is classified as a sapphire and the most sought-after rubies are of the hue described as "pigeon's blood red". With a Moh hardness of 9.0 the corundum stones in general are only slightly less hard than diamond, but enough so that they were workable with pre-modern methods.
Prior to the machinations of the De Beers cartel, rubies were considered the most valuable gemstone to be had, carat for carat - rarity may have played some part in this as the only truly rich source of rubies on earth is South East Asia (especially Burma). The value of any given ruby can also be increased by the presence of an asterism - a pattern of impurities that cause a star like pattern to appear within the stone in certain lights. Historically red spinels, tourmalines and garnets could also be classified as low grade rubies1.
Mystically the ruby is associated with energy, courage and all forms of passion, as well as (somewhat predictably) fire and blood. It apparently corresponds to the astrological houses of Aries, Cancer, Leo, Scorpio and Sagittarius, is ruled by the Sun and marks fortieth anniversaries.
In modern times, ruby lenses can be used in the construction of some kinds of laser.