Silicon is one of the most prevalent elements in the universe. It's also abundant in our Earths crust. However, it's mostly in dust or powder form so it's rare that you find a natural stone of the substance.
Silicon has many uses, but one such modern use is for the manufacturing of integrated circuits. The computer you are viewing this is reliant on the properties of silicon to form microscopic circuits. As for my rock collection, I've acquired a few small scraps from the ingots used to create IC's. It polishes to a high, chrome-like finish.
For more on silicon, click here.