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Feldspar grows (when allowed to) into rectangular crystals, and then usually breaks the same way. This breakage pattern is called cleavage, and feldspar has two directions of cleavage that meet at close to a 90 degree angle (just like a stairway). If you see little sparkles when you look at the rock, the flashes may be the light reflecting off the mirror-like cleavage surfaces.


Quartz has no cleavage (it breaks with a conchoidal fracture - just like obsidian, or the curved fractures that you can often see in the windshield of your car after a rock hits it), so usually no mirror-like flashes of light. The bad news is that sometimes the curved conchoidal surfaces can flash as well, but this usually happens with the clear varieties of quartz (which you already know isn't feldspar anyway).


So, if the mineral has cleavage and sparkles, it may be feldspar. If no sparkles, no matter how you rotate it in the light, it may be quartz.

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