I know that chickens need grit. And I know once they start laying that they need grit and oyster shells(correct?). The grit I have been giving my girls says for up to 10 weeks. It is made from granite. They are about 4 months now. (I know that that is older than 10 weeks). Sooo, what should I be buying for them? My local feed store sells all different kinds of grit but it is in bulk and does not have any labels to read. And when I asked them, they didn't know what I was talking about and told me to give them oyster shells....so am I wrong? Also, at what age should I start giving them oyster shells? Or should I wait until they start laying to give that to them? Thanks for the help!