Join Date: Jul 2004
Location: Gadsden, Alabama
Speaking as a geologist with some background in geochemistry - at aquarium conditions, quartz is insoluble for all practical puposes. In any case, any water but rainwater would already be at saturation point for quartz, it is a ubiquitouos mineral.
You ask if any rocks or mnerals (rocks are assemblages of minerals, BTW) will lower pH - the answer is yes. Any that contain pyrite, a common mineral will do that; the pyrite breaks down to form sulfuric acid. Actually, another iron sulfide, marcasite - also pretty common but much less so than pyrite - is more of a problem in that regard, it is not stable in oxidizing environments. Pyrite, also knows as fool's gold, is usually well crystallized, forming cubic crystals that are metallic yellow; marcasite is silvery yellow and more likely to be found ih coal bearing rocks, a bad bet for aquaria anyway! Weathered pyrite forms rust stains.
BTW, some good tests for quartz: it readily scratches steel, and breaks with a fracture that resembles the lines on a clam shell Thatfracture is best seen on a crstal rather than on massive quartz.