It is from the class of rock most prevalent on earth. Quartz is just one way. The silicates are multiple and variable. Chert, agate, jasper, onyx, agate are all silicates and many of the hardest gems are silicates. Look at any beach anywhere. The sand is most likely ground silicate by weather, water, sun and time. The quartz forms under varying conditions from the same substance (SiO2). for one set of circumstances, the silicone dioxide forms shirt and under another it forms quarts. Sometimes, The courts forms a different type of courts because other trace elements are present. Amethyst are formed Buy one such set of circumstances. Sometimes if titanium or chromium or beryllium our present then you have the formation of sapphire or ruby or Emerald. They only form where the trace elements and the proper pressure and temperatures are present.