These rocks aren't going as further as the ones found in a trash can in Liberty City, years ago.
Vangelico is a robbable store located in Rockford Hills. Your objective is to steal jewelry and escape to the hideout without getting arrested. 
Jewelry store will be displayed on the map with an orange diamond icon:
Head to the checkpoint at the counter and type /robbery to start the heist. You must break the cases around the store with a large weapon (SMGs, ARs, etc). Steal as much as you can, since it's not compulsory to break all the cases. However, the more you get, the bigger the reward.
Once you've robbed the cases, head to the hideout marked on the map.
When you arrive at the hideout, the robbery will be complete. You will receive a decent bonus if you managed to rob all the cases.
Arriving at the hideout without cops near you will decrease your wanted level.