Quartz is a mineral composed of silicon and oxygen that can have large or small crystals. The microcrystalline quartzes have crystals too small to be seen without magnification. Chalcedony is a microcrystalline quartz. Agate and jasper are both varieties of Chalcedony.
Like all quartz, Chalcedony is a hard mineral. On the Mohs scale of 1 to 10 ( 10 being Diamond ) it has a hardness of 7. Chalcedony occurs in a wide range of colors. The most popular are the blues and greens but it can also be found in white to black, yellow, brown, peach and orange, with or without bands or patterns.
The primary difference between agate and jasper is whether or not you can see light through them. Agate is a translucent to semitransparent. Agate is generally banded or has inclusions that look like moss or feathers.
Agates form in areas of volcanic activity where waters flow through open veins and cavities in the rock. When the water has a high concentration of dissolved silica, it can crystallize to form quartz.
Impurities in the water can alter the the color or produce banding, plumes or moss that are often seen in translucent agate.
Jasper is an opaque form of chalcedony. Jasper contains enough impurities and other material that light and images cannot be seen through a thin slice.
Commonly, Jaspers form when soft sediments are deposited in large shallow beds and,with Jaspers, the silica solution acts more like a cement to bind these materials together. Jaspers are also formed when volcanic ash is cemented by the silica solution.
On the left, Blue Chalcedony, on the right top-to-bottom Blue Lace Agate and Leopard skin Jasper.