Ruby is a gem added by GregTech, along with Sapphire and Green Sapphire. It cannot be used interchangeably with the RedPower 2 Ruby, but it can be converted to a RedPower 2 ruby provided that there are at least 9 to create a Ruby Block that can then be placed in a crafting grid to create 9 RedPower rubies. Without being converted, GregTech Rubies can be used in the creation of an Energy Crystal or a Mining Laser or the creation of Ruby Dust.
The Ruby item (see picture of ruby above ore) is no longer obtainable in mindcrack pack v8.0.1 and all ores of the gregtech ruby shall become the redpower ruby (including the already mined gregtech ruby). Other modpacks need confirmation.