How can you tell a diamond from a piece of quartz rock?

Diamonds are a LOT harder. Quartz is hard too, and will also cut glass. Try it against something harder, like a diamond.

The quartz crystals are hexagon prism. The diamond crystal are octahedron (8 triangular faces) but it is hard to find a perfect crystal. Usually one can find the diamond a little rounded (the shape would be between the octahedron and a ball). The diamond is harder than anything so it can scratch easily glass and quartz. The quartz wont scratch another quartz.

First, a quartz crystal will have a very distinct hexagonal cross-section with defined planes. A diamond found in nature will look like a lump of translucent rock. It will not have facets. Second, quartz has a hardness of 7.0 on Moh's scale. Anything over 5.0 will scratch glass.

Where did you find you "diamond"? Diamonds require very specific conditions to form. It takes extreme heat and pressure to cause carbon (C) to liquefy and crystallize in the tightest possible molecular frame known to man. They usually require magmatic or volcanic structures and forces like kimberlite pipes or dykes to form. There are a few places in the US where diamonds are found, but rarely. Quartz, silicone dioxide (SiO2) is very, very common. SiO2 makes up many of the rocks you would find around your house. Sand contains a lot of quartz. Quartz crystals are much easier to form and can grow quite large. The other answers are good. Quartz will scratch glass, but I am not sure on the hardness of a steel penknife blade. The cobwebs in my mind say it is a hardness of about 7. If you penknife scratches it, then it is quartz.