Quartz countertops are approximately 94% ground quartz which is mined much the same way as granite. The remaining 6% is a manmade resin to bind the ground quartz and pigments to give it color. For some designs, small amounts of recycled glass or metallic flakes are added to the mix. The resins also help make quartz counters stain and scratch resistant and non-porous. Although quartz is mostly advertised as not needing to be sealed we do seal upon installation and recommend the same sealer maintenance as with granite. Better safe than sorry!
The biggest difference between granite and quartz is that quartz is NOT heat resistant. You must put down hot pads when used in the kitchen and it is not safe to use outdoors.
Quartz has a huge variety of colors and patterns and it is very easy to get identical slabs. Quartz can look nearly indistinguishable from marble or it can be made fire engine red - there are no limits.