Eogan describes some "spreads" of exotic stones , quartz ,granite and banded mudstone at the entrance to Knowth as well as a saucer shaped setting 4.5 m from the kerb which included quartz , sounds like it may be same thing .One of the properties of quartz is thermo-luminescence which can provide "earth-lights " which must have been noted and wondered at plus even striking two bits of quartz can provide light . It's proabbly easier to mention the sites that didn't have quartz .