While it's not possible to make the crystals into a vessel, if you'd like to play with memories, the hallucinations could be a memory and they could view a "memory" through that method. However, these crystals are only used for healing so they would not have any properties for containing memories.

That said, there may be other methods to store or record memories. They can explore spells that involve crystal balls, mirrors, or scrying bowls if you wish. Yennefer and Stephen Strange could be involved since they've studied magic at the castle. We would recommend this be submitted as a Plot Engagement!