I received a stone while picking fruit (specifically Papaya). Crew Edit: I have edited the guide to reflect this information, and you've been credited! Thank you so much for your help! :) - SerpentEye
As to the amount to put in the treasury, that depends on how long you are going to leave it without replenishing. Note that you will have to visit on a regualkr basis to keep the citizens happy and productive. With both quests complete, the kingdom burns $75,000/day (and is the most productive). The maximum that can be deposited is $7.5M