I think immortality in The Sims 4 just prevents death by old age. I don't think there is any way to prevent "accidental" deaths except avoiding the situations that cause them.
Vampires, who have natural immortality, can still die from sunlight and other causes. Spellcasters who drink the Potion of Immortality will still age up to Elder and can still die from all other causes except Old Age. There is another Spellcaster potion, Instant Resurrection, that brings a Sim back to life immediately, but even that doesn't prevent death. I don't have Crystal Creations, so I can't test hematite carvings, but I suspect they are the same way.
