Privacy Policies - Smart contracts running on blockchain potentially disclose all data to the participants of the chain. Therefore, because privacy is important in many areas, smart contracts may not be considered a good option. To overcome this limitation, this paper introduces Stone, a privacy preservation system for smart contracts. With Stone, an arbitrary Solidity smart contract can be combined with a separate privacy policy in JSON, which prevents the storage data in the contract from being publicised. Because this approach is convenient for policy developers as well as smart contract programmers, we envision that this approach will be practically acceptable for real-world applications.