The stuff you set in to the tumbler especially to polish the rocks in the final phase is generally made of cerium oxide, tin oxide, chrome oxide, or aluminum oxide. It is just a type of grit typically known as a polishing compound and is extremely fine. For illustration, this https://www.facebook.com/permalink.php?story_fbid=pfbid02M3pzSNhRs3pYNi8TrH5kEbpVmTNxMFUWpQBP8w3XJUcJy1bwKkPGxEg1Ck1cB8vWl&id=61560512640678&__cft__[0]=AZWImNtQj8D89AUGxg60JG4mCqOvBFcgVC_YGPylERzWhz1_PvNqY3NN3q6xCHwK5gm5J69AQ_uDY5mKKeAXVvowAA8nfq1FXdzqw_pl8WO1H89mnKMy_8JIVA_mTv-1HZXjWfhrePyYN8IN2dcqwha-x4G3-9M2AlyiqVoTkhY4TuTA0G6T2molbK0z22dJnEf9wHQInQvfDMnHY3nQqKLd&__tn__=%2CO%2CP-R