Currently, you can't lose your Crystals by taking part in staking.

However, there are peculiarities of DePool validators that may create risks of losing your Crystals when staking in the future.  

Each DePool is linked to one of the Free TON blockchain validators. Validators generate new blocks of the network, and the security of the blockchain depends on the quality of their work. 

To improve the network stability, it is planned to introduce slashing in the future — a system of penalties for incorrect operation of the validator. So, if the validator didn't have time to sign the transaction, missed block generation, or made other mistakes, it gets slashing points in the range from 0 to 1. When the score exceeds 0.66, a penalty amount will be deducted from the validator stake.

First of all, the penalty will be charged from the validator's funds — Validator assurance. But if the amount of the penalty exceeds the amount of the assurance, the remaining amount will be charged in equal shares from the stakes of all DePool depositors. 

Slashing score information for each validator is available at 

Although slashing is currently only being tested and does not affect validator stakes, slashing scores are very useful when selecting staking DePool. 

Read more how make the right choice of DePool.