Trachoma calculus is mainly caused by infection with Chlamydia trachomatis resulting in calculus formation. It is not actually a stone or calculus in the true sense, but rather a coagulum accumulated in the depressions or crypts of the conjunctival epithelium, consisting of desquamated epithelial cells and degenerative cells. Thus, it's not a real calculus, and is commonly seen in adults, primarily occurring in patients with chronic eye diseases such as trachoma and chronic conjunctivitis.