Ruby

Ruby is a gem added by many various mods. They are used for tools, Energium Dust and Ruby Dust is the main source of Chrome and Aluminium.

In Project Red, Blue Power and RedPower 2 Ruby tools are the tier between Iron and Diamond. While they have the speed of diamond, they only have 501 uses, and ruby pickaxes can't mine Obsidian.

In 4-Space, Ruby tools have a higher mining level than Diamond, but are slower and less durable. Ruby armor, on the other hand, is only marginally less durable than Diamond, and has the same rating.

Rubies can occasionally be found in Dungeon chests.

GregTech
Fool's Ruby is a gem added by GregTech. It is visually identical to an actual Ruby, but Fool's Ruby components cannot be used in any recipes. Fool's Rubies can be differentiated from real Rubies by checking their metadata: the metadata value of all Fool's Ruby items are 10 higher than their real Ruby versions.

Engineer's Toolbox
Ruby is currently not used, but it will be used in the future.