Put on gloves and a dust mask. Pour some acetone onto the crack in the limestone and scrub gently with a brush to remove any grime or dirt. Rinse with water and let dry.
Spread epoxy adhesive over the crack and push the two pieces together. Latch a clamp around each piece to hold them together, if possible. If not possible, hold together by hand.
Wipe away any excess epoxy with a rag. Let the epoxy adhesive dry according to the manufacturer's suggested drying time.
Clean the limestone with water and dish soap once you fill the crack. Buff to shine with a microfiber cloth.