Diamonds are the hardest substance in nature. They are the result of intense heat and pressure. Most form at least one hundred miles below surface. Some form as far as four hundred miles below ground. famous gems in the world One of the most is the Hope diamond. It is a rare, blue-gray color. It may have been cut from a diamond owned by a French king in the 1600s. The Hope diamond has had many owners over the years. It was even missing for a while after a robbery the owner. He In 1958 Harry Winston was gave the gem to the Smithsonian. In spite of its great value, he simply mailed it to the museum. The large diamond is still one of the museum's most popular displays today.