irreplaceable

Adjective

not-comparable

That cannot be replaced, especially because it is unique.

Japanese Meaning

置き換えることができない / 代えの効かない / 唯一無二の / かけがえのない

My grandmother's handwritten letters are irreplaceable to our family.

祖母の直筆の手紙は私たち家族にとってかけがえのないものです。

Sense（1）

That cannot be replaced, especially because it is unique.

For some purposes lead is unique and, apparently, irreplaceable. Battery plates may be an example of this.