Clear quartz is the plain version of a mineral family that includes amethyst and citrine, and it is one of the most abundant minerals in the crust. Gem-grade material is transparent; most of what goes into beads is translucent, with internal veils and small fractures.
On a natural strand those inclusions are visible and vary from bead to bead, which is the simplest way to tell natural material from glass: glass is uniformly clear and often carries tiny round bubbles instead. The 6 mm size has more beads to the strand and therefore reads slightly denser on the wrist.