A Vossius ring is caused by blunt force trauma to the eye, resulting in the compression of the iris against the lens.
In more detail, a Vossius ring is a circular imprint of pigment on the anterior lens capsule of the eye. This occurs when the pigmented cells of the posterior iris epithelium (and/or the pupillary ruff) are forcefully compressed against the lens capsule during an episode of blunt ocular trauma. The impact leads to the rupture of these pigmented cells, releasing melanin pigment. This pigment is then deposited onto the lens capsule, creating the characteristic ring-shaped appearance.
Essentially, it's like a "stamp" of the iris being left on the lens due to sudden impact.
