irreplaceable
adj.describing something or someone so special or unique that they cannot be replaced if lost.
adj.impossible to replace if lost or damaged; unique in value or significance. Often used to describe people with unique skills or items of immense sentimental or historical value.
“This old family photo is irreplaceable.”
“The museum lost several irreplaceable artifacts during the flood last winter.”
“While most equipment in the laboratory can be reordered, the decades of handwritten research notes are entirely irreplaceable and must be stored in a fireproof safe.”
Typically used as a predicative adjective after 'be' or 'seem', or as an attributive adjective before a noun.
While 'un-' is a common prefix for negation, 'irreplaceable' is the standard form for this specific word.
“They are irreplaceable records of a living world that is changing faster than at any time in recorded history.”
“When he was replaced by a bigger name in a transferring show, he decided to make a production in which he was irreplaceable.”
From ir- + replaceable.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.