Very high pressure turns ordinary coal into diamond. Explaining how diamonds are made would mean writing a book and believe me, there are a lot of books out there that discuss this. T he size and shape of the diamond determine its price. Diamond size is measured in carats. The bigger the diamond, the more expensive it is.
Shape is a preference.
T he rule of thumb for diamonds is literally the 4Cs . The four Cs actually determine the quality of the diamond. The higher the weight, the higher the value. Refers to the shape, proportions and finish combined to create the sparkle . More sparkle means more value.