1 Paris: You bought helium as a short-term investment?! It’s the second most abundant element in the universe!
2 Serron: I bought a valuable isotope.
2 Spanners: Wait. Helium-3? That’s actually sensible.
3 Serron: No. The even rarer and more valuable Helium-5.
4 Spanners: That has a half-life of 600 yoctoseconds! That’s less than a quadrillionth of a nanosecond!
4 Iki Piki: He did say it was a short-term investment.

Helium has two stable isotopes: helium-4 with two protons and two neutrons, which makes up virtually all of the helium found on Earth, and helium-3 with two protons and just one neutron, which accounts for only a little over one millionth of the helium on Earth. The relative abundances of these two isotopes vary a lot in different parts of the universe. In interstellar space, helium-3 makes up over a ten-thousandth of all the helium.

Besides being much rarer, helium-3 is also potentially useful as fuel for fusion reactors, the long-promised clean and safe energy source of the future. So in a futuristic society, helium-3 is likely to be pretty valuable.

There are other isotopes of helium with different numbers of neutrons, but all of them are radioactive and decay to other isotopes/elements. Besides that, they all have short half lives, less than a second. Helium-5, with three neutrons, has a half life of 602 yoctoseconds, or 602×10^-24 seconds.