They were formed deep within the planet, approximately 90 to 120 miles below the surface, at temperatures and pressure difficult to imagine. We never would have had the opportunity to know these amazing creations if nature hadn't intervened, forging paths to the earth's surface through which the diamond-bearing material (kimberlite or lamproite) could pass. Over time, that material was eroded and carried away into alluvial, littoral, and marine sources--all what geologists refer to as secondary deposits. Secondary deposits were the first to be worked by ancient cultures. Although there are thousands of kimberlitic and lamproitic pipes, only about 15 percent contain diamonds. Of that 15 percent, only about 5 or 6 percent are commercially viable. That equates to about two dozen working mines to meet the world's demand for diamonds.
During its rigorous trip from deep within the earth, diamond-bearing material that was more heavily included would break off and wear away, leaving behind cleaner, more durable pieces. Furthermore, marine deposits (offshore) contain some of the world's finest diamonds due to millions of years of wave movement, which has naturally done away with the weaker, more included stones. Perhaps that's what gives sand its sparkle...
Though diamonds are essentially just carbon (99.95 percent)--which is quite phenomenal if you think about it--diamonds do not exhibit any of the phenomena that some gemstones show.
Most of the world's diamond production by volume is only suitable for industrial purposes. An extremely small percentage of the world’s total diamond supply is clean and colorless to near-colorless, making it suitable for jewelry. Approximately one ton of rock is mined for every half carat of diamonds brought to market. To learn more about a diamond's journey to jewelry, read Diamond, King of Gemstones.