The recipe that i've been using is 5 cups oyster shell about 1 1/2 cups I/II portland and 1 1/2 cups water. My rocks are turning out great but the oyster shell that i have bought comes in 5lb bags and its 8 bucks. All the feed stores around here sell what they call hard shell which is calcium carbonate. I thought i found a store that sold 50 lb bags of crushed oyster but when my husband got home with it, it also has silica Quartz in it and i was wondering if i could use either of these, I can get them for about 7-9.00 for a 50 lb bag?