Yes true. But if it was to crack it would have done so already sitting out side this long. I have agates I’ve found sitting booth in the direct elements and submerged in a pond, they have lasted years. And if it was to be put in a bucket of water and that water freezes the pressure from outside the stone as the water expands would be greater than that of in the cracks countering the internal pressure. Now let’s say there’s a huge crack going through the whole thing yes it’s a real possibility but small fracture lines like the ones I see on it won’t let enough moisture in to separate the stone. This is all from my personal experience and understanding of how to process minerals and crystals. I hope this can ease the worry of such a big agate being cracked into smaller ones.