I can't confirm a shell fracture because I can't see any fracture surfaces on either stone. There's a possibility it could be chert, but chert isn't often found in the Koyoshi River system in Yurihonjo City, Akita Prefecture, where these were collected. Both stones have cracks, but they appear to be cracks that won't easily break. For those who can't travel far, I'd like to offer these unusual stones at a low price (lol). As a bonus, I'll include other unusual stones like petrified wood, so please look forward to it. Sizes: Smooth Stone: Length (longest part): approx. 6.5 cm Width (longest part): approx. 6.7 cm Thickness (longest part): approx. 2.7 cm Weight: 138g Rough Stone: Length (longest part): approx. 7.6 cm Width (longest part): approx. 6.4 cm Thickness (longest part): approx. 3.6 cm Weight: 180g Shipping will be via Kuroneko Yamato Takkyubin (Yamato Transport), Compact size.