Jackstone calculus is the name assigned to the appearance of a subset of urinary calculi, a rare bladder stone with spiculated and stippled edges 3.
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Pathology
Jackstone calculi are almost always composed of calcium oxalate dihydrate. They are nearly always created, and thus, located, in the bladder rather than the upper urinary tract and are usually due to bladder outlet obstruction such as an enlarged prostate 4.
The composition of a loose crystalline lattice of calcium oxalate dihydrate, rather than calcium oxalate monohydrate 4, renders them more susceptible to fragmentation with various forms of lithotripsy, though the downside is that lithotripsy is less efficacious for bladder stones 2.
Radiographic features
The spiculated edges of these stones resemble the appearance of toy jacks, hence the name.