Agate versus jasper: the translucency test

Both agate and jasper are cryptocrystalline quartz, meaning the individual crystals are too small to see without magnification, but they are usually easy to tell apart with a flashlight. Hold a thin edge of the stone up to a strong light source, agate is translucent and will let light glow through, often revealing its characteristic banding in the process. Jasper is opaque and blocks light completely regardless of how thin the edge is, because it contains more impurities packed between the quartz grains. Both share the same hardness (around 7) and the same basic fracture pattern, so translucency really is the most reliable quick test between the two.