Pebbly stool indicates the need for more fiber in your diet. The black color could be caused by internal bleeding, iron supplements, or eating dark or blue foods such as blueberries, black licorice, or bismuth/ Pepto- Bismol.

No offense but I'll take your word for it that it resembles papaya pit. Is it one piece or lots of small pieces? And how big is it? Have you experienced it again since the first two times or has the problem gone away?

To be certain, you can always submit a stool sample for laboratory analysis.