Agates are a microcrystalline variety of chalcedony (about 6.5-7 on the mohs scale) formed from layers of quartz. They may occur as rounded nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the breaking away of the volatiles in the molten mass.  These cavities were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. 

This type of vein agate, when cut transversely, exhibits a succession of parallel lines which may be quite thin and slender, giving a banded appearance to the agate.

In the formation of an ordinary agate, perhaps those found in or near streams and rivers, it is probable that waters containing silica in solution percolated through the rock and deposited a siliceous coating on the interior....variations in the character of the solution or in the conditions of deposit may have caused the corresponding variations in the successive layers of agates....i.e. bands of chalcedony often alternate with layers of crystalline quartz.

The outer surface of an agate is rough, pitted and ugly and it masks the beauty of the stone inside.  However, the crust is weak and somewhat fragile and over centuries it is washed away allowing the gemstone to be discovered.

Agates in general are very plentiful and are found all over the world where highly pressurized hot water rich in silica filled crevices and vugs.......agates come in every conceivable color and type....but some are known for their incredible color and formational designs.

Many agates have been named due to their location.....Mexican Flame, Botswana, Montana Moss, Russian etc. while others were named to describe their designs....Tree Agate, Banded, Blue, Brecciated, Green Pom Pom etc. 

Often tiny quartz crystals (drusy) form within the stone and add to the beauty and uniqueness of the individual material.