Yes, after the Law of definite proportions; but now it is clear that this law is not applicable to all known chemical compounds.
No. Minerals have definite chemical structures, but not rocks.
No. Granite is usually made up of three main minerals - Quartz, Feldspar and Mica. These occur in random quantities that make each granite different.
Minerals have a definite chemical composition, are solid, are inorganic, are naturally occurring, and have a crystalline structure.
Water, H2O, is a definitely a chemical composition. There are many many others!