He decided to tell Jackie the truth.
He explained how he had first found the powers of the stone, how he had experimented, and how he had resolved to try this experiment. Far from being cross, Jackie's first move was to pull Arnold away from on top of her, and put on the light. She wanted to see for herself. It was true, she had a penis of her own, a bit limp after the excitement, and a pair of aching testicles. This gave her a slight erection, but she decided to return to bed, and cuddled Arnold. "Let's keep them until the morning" she said.
When Arnold awoke in the morning, Jackie was already in the bathroom. She had showered, having carefully soaped and washed her new genitals. She was now fully dressed and was singing as she put on her makeup. "I want to keep it a little longer" she said playfully, as Arnold put his head round the door. Why shouldn't they? If it meant that tonight they would have another romp, then... It was decided. Jackie explained how he should carefully clean his now tight little pussy. She also put a sanitary towel in his underpants explaining how last night's fun could leak out for some hours to come. He dressed, realizing how much more comfortable it was not to have that sack between his legs. They breakfasted with a new air of glee between them. Arnold was looking forward to the evening.