Answer of first question
Gemology is a science dealing with the natural & artificial materials.It is considered as a Geoscience and a branch of mineralogy. In other words,study of natural, Synthetic, composite, reconstructed as well as imitation gem materials.
Stones which are formed naturally beneath the earth crust are called natural gemstone. Gemstones can be organic as well as inorganic.
Any gemstone, completely or partially manufacture by human being is known as synthetic gemstone. The physical and chemical properties and/or their crystal structure essentially correspond to their natural occurring counterparts (natural gemstones).
There are basically three qualifying parameters for gemstones, these are - "Rarity","Originality","Durability".
Durability simply means the resistability of a gemstone against dust, shock, Chemical & light.
Any gemstone, artificially manufacture by Composing/assembling of two or more natural gemstones/ synthetic gemstones/other minerals/any other material, known as a composite stone.