Jasper is another of the cryptocrystalline varieties of quartz, but unlike its first cousin agate, jasper is a "cemented product." Instead of silica forming in a nodule or crack, the silica solution slowly permeates a sedimentary material. Over time the silica acts like a glue, cementing the sediment together forming the hard material we call jasper.
In the case of jasper the color often comes from the original layers of sediment and not necessarily from staining caused by the mobile silica solution. Secondary features do sometimes appear in jasper like dendrites, which can form in narrow cracks or displacements in the main mass. In this case they are made up of the same materials one might expect in an agate. Cracks in jasper can be later filled by agate producing mixed stone varieties.
The size and composition of the sediment helps to determine the "hardness" of the final jasper and much of its color. It is also responsible for the ability of the jasper to take a polish. Very fine grained mudstone or siltstone can produce ultra fine particulate jaspers that result in high polish and nearly invisible grain. These are sometimes called porcelain jaspers and are among the highest quality.
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http://www.theimage.com/newgems/quartz/jasper/I miss Ron. He was a wealth of information.