Diamonds are a Man's Best Friend!
Diamonds are pure carbon crystalline stones formed under extreme heat and pressure within the Earth's crust. The MOHs hardness scale puts talc at zero (softest) and diamonds at 10 (the hardest know substance on Earth). Knives sharpened on stone will eventually wear out both surfaces. Tiny steel particles can be seen in the on the stone and over time the stone's flat shape changes from the steel being sharpened. Later the stone will require "truing" which means making it truly flat once again (unless you want a knife with an uneven edge). For most purposes stone is great for sharpening edge tools and they are relatively inexpensive. However, I learned recently from experience if my waterstone grinding wheel is not completely flat (true) it will ruin the surface of my knife, leaving me to work much harder to fix it. To remedy this, a diamond cutter is used to reshape the surface of the stone back to it's original flatness in a consistent manner. The problem is this, the grinding stone gets smaller and smaller. Diamonds are forever, they say. As the hardest substance known to humans you cannot wear down a diamond, making it the ideal material to maintain and sharpen edge tools like knives. Since natural diamonds are rare, they are expensive. Industrial diamonds are those diamonds too ugly or with too many inperfections to be used for jewelry. Also diamonds can be made synthetically. In either case diamond grinding wheels are much more expensive but worth it since they last long and never need truing. Definitely my best choice!