Grit on a granite countertop is usually due to poor quality granite which cannot be solved. Sometimes it is grout particles flaking off from newly grouted backsplash, but that's not too common. Also, if a sealer was applied incorrectly or left to dry on the surface, then the surface may feel dusty or even a bit sandy and may look hazy from the sealer residue on the surface (which will not occur when a sealer is correctly applied).
See these Q&A articles for further discussion:
Kitchen Countertop Isn't Smooth After Install
Here are a few other explanations that we've heard from fabricators (and our assessment):
"It's a porous granite and we are feeling the pores." (bogus)
"A mud based cutting system may have been used and the mud gets into the granite and seeps out." (bogus)
"It's the mica and will go away in time."(possible and worth considering, but not likely)
This problem is seen mostly with gold-colored granites. This does not mean that gold-colored granites should be avoided. Many gold-colored granites like Santa Cecelia are very popular and never have a problem. However, when a granite countertop does feel gritty or sandy, it is usually a gold-colored granite.
Often it is suggested that the slab just needs more polishing. It's highly unlikely it needs additional finishing or polishing. The reason it is gritty is not because it wasn't finished completely or correctly. If so, it will feel rough, but not like sand or grit is on the surface.
The slab was originally finished and polished on specialized industrial machines which typically achieve the best finish possible for that slab. Meaning additional polishing will not improve upon what the machine has already done, and thus, will not solve the gritty granite issue. Not every slab is equal. Some will polish better than others and some will have a higher gloss shine than others. Poor quality slabs cannot be polished well. Often such slabs will have rough patches or just won't feel glassy-smooth.
Granite is made of "grains". These grains can pop out, which is why some granites will form pits. Gritty or sandy granite is usually just a slab that was not well-formed geologically and the grains are constantly coming loose. Finishing / polishing the slab does not magically hold the grains in place. It just smoothes the surface.
The only potential solution to this is to apply a permanent topical coating. This is not advised since you must maintain the coating which is often more troublesome. The best solution is to completely replace the countertop with quality granite slabs. Poor-quality slabs should not have been installed in the first place. However, if that is not possible, then a coating may be the only solution.