1) The diamond is a symbol of good, purity, wealth, and power. It also has healing virtues. A popular belief was that this gem could cure a disease by simply placing it on a part of the body. (Fun fact: it was later realized during the Renaissance that ingesting diamond powder killed more than it healed).
2) The diamond is a precious stone unlike any other. That's why these stones are expensive. They are rare and there is a lot of demand. It is one of the most sought-after stones in jewelry.
3) There are cultured diamonds, these are synthetic stones. This stone has the same physical, chemical, and optical properties as a diamond from the mines.