Agate is a type of microcrystalline Quartz (Silica) generally associated with Volcanic rocks and normally has a distinctive banding pattern.
The colour patterns and banding of this translucent Gemstone is unique and although there are many styles and patterns with a Hardness rating of 6.5 to 7.0 on the Mohs Scale this gemstone comes in many colours.
Some examples are shown Below

      Agate after polishing       Typical Agate in it's raw state

Agate can come in many colours - here below you can see the equivalent in blue

Blue polished Agate       Here in it's raw state