A、3.3 billion years ago. B、2.9 billion years ago. C、1.2 billion years ago. D、2.1 billion years ago. C原文讲述最年轻的钻石都有12亿年了，由此可知，它们是形

Diamonds are the hardest substance known on Earth and they make sparkling jewelry as well as saw blades that can cut through pretty much anything.
    Recently, geologists studying diamonds have identified the ages of thousands of diamonds from Southern Africa, where the mineral is most abundant. Their research found that there were only three times in Earth’s history when diamonds were made and that Earth no longer makes diamonds like it used to. "Something was different then. Perhaps the planet was hotter on the inside, or the composition of the rocks was subtly different. Whatever it was it has changed now," stated Steve Shirey, of the Carnegie Institution in Washington, D.C., one of the project researchers.
    The oldest diamonds were made 3.3 billion years ago when Earth was relatively young. The second time diamonds formed was 2.9 billion years ago. Small bits of rock within those diamonds, called impurities, indicate that they probably formed from rocks laid down in a shallow sea suggesting that the carbon that made the diamonds probably came from the remains of ancient sea life. The youngest diamonds on Earth are 1.2 billion years old although a few smaller diamonds are about 100 million years old.
    Some people like diamonds because they are pretty, rare, or expensive but Dr. Steve Shirey has a different perspective. "I think of diamonds," he said, "as being tiny time capsules that encase a little piece of rock protecting it for billions of years and providing us with a unique window on ancient times."
19. What does the speaker tell us about diamonds?
20. What does the speaker want to convey by the remarks of Steve Shirey?
21. According to the speaker, when were the youngest diamonds made?