Grit substitute?

[EggWalrus]

I put egg shells n the microwave for 2 min then stick in a baggie & roll with a rolling pin.

Egg shells are not grit. Same with oyster shell. It is a calcium supplement. It is soluble as in it gets ground up and digested.
Grit, whether store bought, rocks, pebbles, sand or dirt, is insoluble. It is used like sand paper to abrade what they eat into a fine mush they can digest and absorb.
What's great about any calcium supplements is you can put out 5-50lbs and if the hens need or want additional calcium they will eat it as needed. But it is not the same as grit that the gizzard uses to grind with. Think of it like taking a bag rusty metal and tossing it in the dryer. If you tumble it long enough, it will eventually clean the metal. But if you put dirt, or grit in the bag, it only takes a few minutes to knock the rust off the metal.
Since chickens ain't got teeth, their gizzard (with the help of some kind of grit) pulverizes their food for them.

 

[centrarchid]

Egg shells are not grit. Same with oyster shell. It is a calcium supplement. It is soluble as in it gets ground up and digested.
Grit, whether store bought, rocks, pebbles, sand or dirt, is insoluble. It is used like sand paper to abrade what they eat into a fine mush they can digest and absorb.
What's great about any calcium supplements is you can put out 5-50lbs and if the hens need or want additional calcium they will eat it as needed. But it is not the same as grit that the gizzard uses to grind with. Think of it like taking a bag rusty metal and tossing it in the dryer. If you tumble it long enough, it will eventually clean the metal. But if you put dirt, or grit in the bag, it only takes a few minutes to knock the rust off the metal.
Since chickens ain't got teeth, their gizzard (with the help of some kind of grit) pulverizes their food for them.

Even the oyster shell can have limited value as grit. If it last long enough in gizzard to help triturate items like seed, greens and insects with shells, then it is grit. It simply does not last as long as grit made of other rocks. Too much black and white yet again.

 

[rjohns39]

To answer OP's question. If you're looking for a substitute or the real thing for that matter, call your local quarry and ask for crushed granite or quartz about half the size of a pea. Alternatively you can go to your big box store and likely find it.

That said, depending on your soil and the amount of rock in it, you may not need anything. I live on rocky soil and quit providing grit more than a year ago. When I process birds their gizzards are full of small rocks.

As to the discussion about grit in feed, some mills use crushed limestone for calcium. It's really too fine and soft to provide any real benefit.