No one has observed before a "free" quark, i.e. a quark in an unbound state. According to one paper, I read that $p\bar{p}$ collision produce unbound $t\bar{t}$ pair which quickly decay into other particles. But some people argue that the produced $t\bar{t}$ pairs decay so quickly that they had no time to bind! And I agree with this idea.
So why can one not find quarks in unbound states?
This is because the force-versus-distance law for quarks is such that the farther away from one another you pull a pair of quarks, the harder they attract one another. It is as if they were attached to each other with a rubber band (a very stiff one!). If you pull them far enough apart, there's enough energy stored in the system to create a new pair of quarks (i.e., the rubber band snaps) which pair up with the others and instead of getting two "free" quarks, you get a pair of mesons with two quarks inside each one.
If you want a more descriptive and a simple mathematical treatment using the formulation of electrodynamics and quantum mechanics, then Leonard Susskind's series of lectures will give you a lot of insights!!
Here is the Link : https://inspirehep.net/literature/1532
Quarks that are lighter than the QCD scale, ($\sim 200\,{\rm MeV}$) will always be confined, but heavier ones can be unbound. The top quark weighs $170\,{\rm GeV}$, so it can be unbound.
The QCD scale is the mass scale that pops out when you renormalize the strong interactions.
