Sandy soil would be better than clay for a dust bath because the clay particles are very dusty compared to sandy soil and breathing in fine dust isn't good for anybody. I know they call it a dust bath, but dirt bath would be more correct. Basically the bird throws dirt into the air and gets it in under the feathers, oils and blechh stick to the dirt and then the
chicken shakes the dirt out. Aside from breathing it in, fine dust may not be ejected from between the feathers as easily.
Oyster shells are used to replace calcium lost in egg shell production so they are good for the birds but they are not hard
enough to maintain a sharp edge in the
chickens digestive tract.
Sandy soil may be enough for grit, it depends on the particle size and composition. I would suggest buying a bag of poultry grit one time to see what it is made up of and then substitute
local dirt to match it. Don't concern yourself with the colour of the grit, just particle size and shape. IE smooth river washed pebbles won't look like the grit but fractured granite bits will.