It is possible that your testicle is hard as a rock due to a condition known as testicular torsion. This occurs when the spermatic cord, which contains the blood supply to the testicle, becomes twisted, leading to decreased blood flow to the testicle and its surrounding tissue. Testicular torsion is a medical emergency and requires prompt treatment. Other possible causes include epididymitis, an infection of the epididymis, orchitis, an infection of the testicle itself, and testicular cancer. It is important to seek medical attention right away when noticing any changes in testicular size or hardness.
It is also important to note that testicular hardness is normal and can result from activities such as exercise, sexual activity, and even cold weather. Therefore, if your testicle has been hard for a long period of time without any other symptoms, it is likely not a cause for concern.