is emerald the most expenses?

Answer 1

A large alexanderite will cost far more than the same size diamond. As will a flawless emerald.

Answer 2

diamonds are generally more expensive, but a good, clear emerald (or sapphire or ruby) will be quite expensive, and can be more than a diamond of lesser quality. That's why those 4 stones are often referred to as "precious" as opposed to "semi-precious," which include stones like amethysts and topazes.
However, a big stone with good color and few/no flaws or inclusions will be dang expensive, no matter what kind of gemstone it is.

Answer 3

I think that the diamond is.