Gneiss is (mostly) metamorphized granite and contains as much silica as granite does. It is only the arrangement of the constituent minerals that is changed by the metamorphosis (heat and pressure cause like minerals to form bands, similar to the stress of work hardening does in alloys), the actual chemical constituents remain the same. You do also get Dioritic Gneiss but even that is still primarily silicate material.
Suggestion: Gneiss should be a silicate source for sand making, or change Gneiss to Schist or Phylite.
I 
rocks,
Lord_PrettyBeard