Can't find poultry grit ANYPLACE in Las Vegas for my inaugural Gritty batch.

Soooooo...... I guess I have to resort to crushed granite.

If I screened out the 1/8" - 1/4" of the stuff below in the pic, do you think that would work? Notice the ruler scale at the bottom of the pic. I really want to get as close as possible to the intended Gritty ingredients and don't want to cut corners.

Also, I am a city boy. Can somebody explain to me why the F chickens eat rocks???

Thanks for any input. :)