A VERY CURIOUS THING. “It is difficult to isolate a single quark, the reason behind this is that quarks never exist by themselves, they always exist in groups. Quarks are basically of 6 types: up(u), down(d), strange(s), charm(c), top(t) and bottom(b) in which up(u), down(d) and strange(s) are three main types of quarks. Nucleons, we know are mostly made of up and down quarks but in nuclei of strange matter, strange quark is also present and they have very high density.”
https://link.springer.com/article/10.1007/s10509-018-3335-9