In this diagram of SiO2, you can see how a molecule of quartz is actually formed. The white atom in the picture to the left is Si (Silicon) while the blue atoms are Oxygen. You can see how even though, the formula is SiO2, there are four oxygen atoms connected to the one Silicon atom. This is because in quartz, the shape is repeated over and over causing all the atoms to attach to one another. The shape of a quartz molecule is tetrahedron. Below, included, are some pictures of the SiO2. In these pictures, you can see how each molecule can connect to many more to form the structure of quartz.