Here's information about capacitors from wiki;
"A capacitor (formerly known as condenser) is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When there is a potential difference (voltage) across the conductors a static electric field develops in the dielectric that stores energy and produces a mechanical force between the conductors." [en.wikipedia.org]
The limestone would be a better conductor of electricity than granite. The granite walls of the KC would act as the insulator and the limestone surrounding it would be one of the conductors. Now if you put another conductor up against the KC walls you would have a capacitor. Say you took something made of copper or gold in there, or maybe a statue covered in a layer of of it. When the statue was up against the granite wall for a while anybody touching it after that would probably get blasted like with the Ark of the Covenant.