The primary reason was that they couldn't let anyone discover the anomaly in the stone; otherwise, a stone that couldn't be cut would certainly cause a worldwide sensation. That's why the robbers arrived so opportunely.
They would absolutely not give up on that peculiar stone. In the end, they would definitely resort to extreme measures to take back the stone. It was only at the last moment, in a flash of insight, that she figured it out. That's why she changed the price at the last minute. Otherwise, she would be the one worrying now, having to confront those people directly.
If her guess was correct, the buyer of that stone had a flight at one o'clock tomorrow, so they would certainly make their move to steal the item during this period. Once they left the venue and the item was robbed, especially since the venue had already been hit by robbers, no one would suspect them, and they would have successfully cleared themselves of any involvement.
