You are most familiar with silica in the form of sand. Glass products are made from basic silica plus other materials. But crystal is also silicon dioxide therefore it is also quartz. While all crystal is quartz, not all quartz is crystal. It actually grows in the ground, taking sometimes billions of years to form.
Crystals grow deep within the Earth's crust, usually around volcanic and earthquake activity.
The process requires immense heat and pressure and always a 'seed' crystal is needed to start it off. This seed is created when a single silicon atom, under intense heat and pressure, fuses with two oxygen atoms from superheated water or steam trapped in the same space. The atoms fuse to form a single crystalline cell of silicon dioxide, the substance from which all quartz crystal is made. Over the millennia, if conditions are right, this seed starts to grow. But the surrounding fluid must contain just the right proportions of silicon and water, or pressurized steam, maintained at a phenomenal intensity of heat and pressure for a sufficiently long period of time.