U.S. scientists have discovered two rare types of sub-atomic particles known as quark combinations that are exotic relatives of the more common proton and neutron. 

The discovery was made at the U.S. Department of Energy's Fermi National Accelerator Laboratory in Illinois. 

These particles, named Sigma-sub-b, are about six times heavier than a proton and "are like rare jewels that we mined out of our data," said Dr. Jacobo Konigsberg, a University of Florida physicist and spokesman for the Collider Detector at Fermilab (CDF) collaboration, which conducted the experiment using the world's most powerful particle accelerator.

"Piece by piece, we are developing a better picture of how matter is built out of quarks," he said Monday in a release. "We learn more about the subatomic forces that hold quarks together and tear them apart."

Quarks are the most fundamental building blocks of matter. There are six types: up, down, top, bottom, strange, and charm.

Particles with three quarks are called baryons — derived from the Greek word "barys", meaning "heavy." The most common of baryons are protons and neutrons.

"Our discovery helps complete the 'periodic table of baryons,'" Konigsberg said.

The new particles are extremely short-lived and decay within a tiny fraction of a second.

Using the lab's particle accelerator, physicists can recreate the conditions present in the early formation of the universe, reproducing the exotic matter that was abundant in the moments after the Big Bang.

While the matter around us comprises only up and down quarks, exotic matter contains other quarks as well.