Stone under the flesh of my knee?
Quite recently I fell off my bike pretty badly and suffered a medium abrasion on my knee. I noticed a black spot in the wound back then. Now, after the wound has healed greatly, it still appears as a strange globule. I felt it's surface but it is smooth and even.
What are the symptoms of a stone embedded in a wound? Are there curious cases in which, if the wound escapes infection, the embedded stone simply remains underneath the flesh permanently? Does the body have any mechanism to eject such foreign bodies? Should I be worried?
Photos are attached.