The difference between Silestone and granite countertops is that Silestone is 94 percent quartz, a mineral composed of silicone dioxide in a matrix of resins, and granite is an igneous rock composed largely of feldspar, quartz and other minerals. Both granite and Silestone are very hard substances.
Silestone resists etching in a way that granite doesn't. Granite is porous and needs to be sealed every few years to resist etching and staining. Silestone is nonporous and doesn't need sealing, and it also comes with a chemical that resists the growth of mold and bacteria.
The look and textures of Silestone have also improved over the years. It comes in dozens of colors that aren't found in granite. However, granite also comes in a wealth of colors besides the familiar salt and pepper.
Granite is desirable because it resists cracks and chipping, but man-made Silestone is even more resistant to this sort of damage. However, if Silestone experiences damage, it's harder to repair than granite. A professional must repair countertops of either material.
Although granite used to be more expensive, Silestone has become a bit pricier as of 2015. However, over time, the ease of care and maintenance associated with Silestone might offset the original price.