Question

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.

Answer 1

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.

Answer 2

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