Gemmology is a branch of mineralogy which covers a vast field. a mineral is a homogeneous substance as it is composed of the same substance throughout. It is formed by inorganic processes and has a definite chemical composition and physical properties which are fairly constant. Its structure is usually crystalline.
Mineral crystals are formed by atoms of elements present in molten rock, under favourable conditions. With time, these atoms come together in closely packed, chemically uniform solids known as crystals. Hence, they have an orderly crystalline structure, definite chemical composition and fairly constant physical properties. These are the same for all specimens of the mineral.