The Nitty Gritty on adding Grit to your worm bins. There are many different items you can add to your bin for grit.
Grit is important to help your wiggly friends break down foods in their gizzard.
 
For most, powdered eggshells are the best option, they help neutralize acidic conditions that may come from adding certain foods and offer great grit for helping the worms digest their foods and make great castings.
You can use a coffee grinder or blender to pulverize the shells.
 
Coffee grounds can also offer grit. Many people worry about the acidity of coffee, but if you are feeding USED coffee grounds, most of the acidity has been brewed out for your breakfast 'go juice!'
 
Sand is one of my favorites, since I use much of my castings for baby plants / seedlings. In my experience, sand offers a casting that has a slightly lighter consistency and tends to drain a bit faster. I see more of a difference in red wiggler castings than those from the African nightcrawlers.
 
I have no experience with 'rock dust' but I've seen several large volume farmers who mention using it regularly.