Natural quartz can emit trace amounts of radon.

More Info: Natural quartz is the fourth-hardest substance on our planet. It can emit trace amounts of radon, much like any other natural stone product used in replacement countertops, tile or hardscape. What is usually called "quartz" and made into countertops is more properly called engineered stone. Engineered stone -- a blend of crushed quartz, glass and resins -- does not release radon because it is permanently sealed during the fabrication process. The vast majority of quartz countertops installed in the United States are engineered, and sold under brand names such as Silestone, Cambria and Zodiaq. Source: http://www.sophisticatededge.com/do-quartz-countertops-emit-radon.html