They do not reset to zero. They are reset to actual points the minor earned by cruising. The status, however, remains. Example: the minor cruised once with Diamond parents on a 7 night cruise. The account would show the minor as Diamond with the parents’ total points. After turning 18, the account would show Diamond with 7 points. To reach Diamond Plus would require earning 168 additional cruise points.
The Crystal Blocks are one per household. My husband has 14 fewer actual points than I have so he would have received a block on the cruise following the one “I” received the 280 Crystal Block if we were each entitled to one.