Alcatraz of the Starways

Venus was a world enslaved. And then, like an avenging angel, fanning the flames of raging revolt, came a warrior-princess in whose mind lay dread knowledge—the knowledge of a weapon so terrible it had been used but once in the history of the universe.
Purple! Mark Denning almost sobbed. A purple Josmian! Forgetting the sweat in his eyes and the insufferable heat about him, his clutching hand held up the mud-dripping globe the size of a baseball, iridescent in the Venusian night.
The phosphorescent glow that bathed the endless swamp in ghastly green, struck myriad shimmering rainbows from the dark sphere.
Two more of those and you're free, lower species! It was an ironic voice, with the resonant sweetness of a cello in its depths, that issued from the haze nearby.
Frantically Mark reached down into the tepid mud, where he had felt the swaying stems of Josmian lilies whip about his knees. Another globe met his hand. He tugged and twisted until it tore from the stem, but when he raised it to the surface, it was white.
Immediately it began to shrink. It would continue until it became the size of a small marble, when it would either rot, as the majority did, or begin to crystallize into a priceless Venusian pearl. But that happened only with one in ten thousand. It was different with the purple ones, they never failed to crystallize into a violet globe of unearthly beauty and incalculable value. Less than a hundred of the purple had ever been found. They were so rare that any prisoner who harvested three, was granted freedom.
Pretty! the cello voice taunted, behind Mark. In a few hours it will be rotting and stinking to high heaven!
Cut it, Aladdo! Mark growled. He tossed the white Josmian into the basket he pushed before him across the mud; the purple one he placed carefully in his trouser pocket. He pushed on, searching the pungent-smelling mud that came up to his thighs.
Suddenly the warm ooze rose to his waist and crept inexorably higher. For an instant, Mark clawed at the mud. It was surging up to his armpits now, as he floundered in the tenacious sink hole. He shook his head to get the sweat out of his eyes and the numbness from his brain. He stopped thrashing about, for he knew that was futile. He threw back his head and gave a shout in which was more than a note of sheer terror.