|Jasper photo by: Ira Bradford. Agate photo by Achat Sphaerolith|
The simple answer is if you put light behind the material and you can see through it, then it is an Agate if you can’t then you're holding Jasper. The more complex answer is that it is not always that straightforward. The simple science behind this question is that both Agates and Jaspers are comprised of Quartz- which is one of the most common minerals on the planet. Quartz is comprised of two major types- macrocrystalline (large crystal) and cryptocrystalline (small crystal).
Now here is where it can get confusing, one major variety of cryptocrystalline quartz is Chalcedony. Chalcedony includes Carnelian, Chrysophase, Agate, Bloodstone, Jasper and others. When Chalcedony in concentrically banded it is called an Agate. Occasionally the banding is larger than the crystal and the banding is not visible- like with most Carnelian.
Jasper is an opaque rock of virtually any color stemming from the mineral content of the original sediments or ash. Patterns arise during the consolidation process forming flow and depositional patterns in the original silica rich sediment or volcanic ash. Hydrothermal circulation is generally thought to be required in the formation of jasper.
The banding in agate is based on periodic changes in the translucency of the agate substance. Layers appear darker when they are more translucent (this may appear reversed in transmitted light). This effect may be accompanied and amplified by changes in the color of neighboring layers, due to other co-precipitated minerals.
The Major Varieties of Quartz (Photos)
Types of Agate With Photos
How Do Agates Form?