About
The Prison Quarry is what the game also calls "the Badongerst Prison Quarry". It lies south of Badongerst, on the road Theodore Badon takes to the South Fortress: "Head west out of Kal Crossing to Badongerst, then south past the prison quarry". The world map marks a Quarry Office beside it, and Penny Tencherries stands close by, speaking for the prison.
What lies under the quarry matters more than the stone. In Penny's words: "My prison just happens to be built right atop a vast network of old mine shafts, rich with all kinds of ore veins." Silvia Bellows recalls "a large cluster of coal veins were discovered in the Badongerst region way back when", and in Persistence that coal becomes Kal Crossing's answer to steel armor: Liza drafts "a trade agreement with the Badongerst Prison to supply you with all the coal you need". Once the deal is done, Penny sums it up: "Coal shipments out, guards in - just the way it should be."
The Jailhouse Rocks event starts here after a rockslide. The event guard has little sympathy for the prisoners ("Those prisoners sure are being whiny about having to climb over a few measly rocks..."), and pays you instead:
Hm? Oh, yeah. We had a minor rockslide and the prisoners are taking forever to clear it out. Production is down because of it too.
... Say, are you interested in helping clear it up? Unlike the prisoners, we'll actually compensate you.
What they say
Watch your step around the quarry. Last thing I need is another visitor in a sling.
Penny Tencherries
Get that contract to your people in Kal Crossing - the sooner this deal is signed, the sooner I can feed my prisoners properly.
Penny Tencherries
Nobody and nothing is placed in Prison Quarry in the game data.