Book 27, Question 40
How long do crystals take to grow in nature?
It depends.
Frost crystals can grow in minutes, and salt crystals can grow in hours or days. Some mineral crystals in caves and rocks take thousands or even millions of years to reach the size we see. The giant gypsum crystals in the Mexican cave may have grown for about half a million years. Diamonds, formed deep in the Earth, took far longer still.