1. A precious or semiprecious red stone which is used in jewelry: "Martha’s engagement ring was made with rubies and pearls."
2. A very hard mineral that is used as an abrasive to rub and to clean surfaces: "The cabinet maker always uses a rubbing tool which is surfaced with rubies so he make very smooth surfaces on the fine furniture which he is making."
3. A gemstone that transmits light with very little diffusion: "The ruby mounted in the necklace was exceptionally beautiful as it was shining on the woman’s neck."