The crystal glows sexily at Jack, pulsing with a deep blue light. When the party ends, he goes home with it.
Jack is fascinating to the crystal, compelling, beautiful. His eyes promise a thousand years of memories -- laughter, affection, taste, touch, love, sadness, loss. The crystal craves memories, pulling them in from those who touch it, refracting them back to give its partners pleasure or pain.
Before Jack leaves, the crystal offers him a gift: it will use its memory-resonance inside Jack's mind, strengthening his happiest memories, blunting the sharpest edges of those that grieve him most.