It's difficult to definitively say a material is *completely* irreplaceable, as human ingenuity is constantly finding new ways to create and use materials. However, some materials are exceptionally rare and have no known perfect substitutes.

Here are a few examples:

* Helium-3: This isotope of helium is extremely rare on Earth but abundant on the Moon. It's a potential fuel source for future fusion reactors.

* Certain rare earth elements: These elements are essential for many technologies, from smartphones to electric vehicles. They are mined in only a few locations, and finding sustainable replacements is challenging.

* Some specific gemstones: Certain gemstones, like extremely large or perfectly cut diamonds, are unique and irreplaceable due to their rarity and beauty.

It's important to note that the term "irreplaceable" is relative. While we might not have a perfect replacement for a material right now, future technological advancements could change that.

For example, we might find new ways to extract rare earth elements more efficiently or develop alternative technologies that don't rely on them.

Therefore, while some materials might seem irreplaceable today, it's always possible that future innovations will lead to new solutions.