A fossil or trace of an ancient belemnite, an extinct squid-like creature that had a hard internal shell shaped like an arrow.
From Greek 'belos' (arrow) + '-site' (fossilized form). Belemnites were common marine animals 200+ million years ago, and their arrow-shaped shells are among the most frequently found fossils.
Belonite fossils look like stone bullets or arrowheads, which is why ancient people called them 'thunderstones' and believed they fell from lightning. They're actually the internal skeletons of prehistoric squid-like creatures, making them one of the coolest proofs of evolution we can hold in our hands.
Complete word intelligence in one call. Free tier — 50 lookups/day.