Identify this very common mystery mineral, which is almost always present in light colored rocks and sandstones: hardness of 7, no cleavage, exhibits conchoidal fracture, glassy luster.

quartz

Quartz is second more common mineral you'll find. Only feldspar is more common. And since most sand is quartz sand, it stands to reason that most sandstone will contain quartz. Despite the very crystalline appearance of quartz, it doesn't have cleavage, and pure quartz does look like glass. It's quite likely as you read this answer that a piece of quartz is within a few feet of you right now. One of its common uses is as a piezoelectric crystal used in creating a stable oscillator used in computers and other devices requiring a cheap, stable, time source.

Quartz is the mineral (with chemical formula of SiO2) which is almost always present in light colored rock and sandstones, hardness of 7, no cleavage, exhibits conchoidal fracture and glass luster. In addition Quartz has a tenacity of brittle and a specific gravity of 2.6-2.7