For 10 points, identify this mineral which is less common than feldspar, and is made of crystallized silica.
Berlinite is a phosphate mineral which has a structure similar to this mineral, and crystals of this mineral which are green are known as prasiolite or vermarine. A variant of it, which usually forms in large, concentric layers, is known as agate. Amethyst and citrine are variants of this mineral, which is the bottom-most mineral on the Bowen's reaction series. It also has a value of seven on the Mohs hardness scale. For 10 points, identify this mineral which is less common than feldspar, and is made of crystallized silica.
Berlinite is a phosphate mineral which has a structure similar to this mineral, and crystals of this mineral which are green are known as prasiolite or vermarine.
quartz [prompt on silica or silicon dioxide]