Oyster Shells let the ladies regulate their calcium intake for strong eggshells.
Chickens don't have stomachs. That was most likely grit they picked up to help grind their food.
New layers can have glitches within their reproductive systems before things become normal. There's actually no yolk in that, just the thick Albumin, & thin Albumin. Glitches can effect shell hardness as well.
One of my new layers years ago would lay double yolkers constantly, one time a...