Gem Spinner Rules
This page has more information about how Gem Spinner and Gem Spinner II work. This information is covered briefly in the in-game tutorial, but there's more detail here.
Why do some matches earn a "tear drop" and others don't?
The goal of Gem Spinner and Gem Spinner II is to clear the board. You clear the board by removing pieces. You remove a piece by making every cell in the piece turn dark. You make a cell turn dark by making a match that involves the cell.
For instance, suppose you make a match of three gems. Each cell (space) occupied by those gems will turn dark (if they are not already dark). When all of the cells in a piece shape turn dark, the piece disappears.
You only get a water drop (or, in Gem Spinner, a step toward your numeric goal) for darkening a new cell--a cell that is not already dark. It's fine to make a match that only involves already darkened cells, and that's good for resetting the timer and getting a few points, but the main goal is to figure out how to darken cells that have not yet been darkened (have not yet had a match made on them) in order to remove pieces and clear the board.
What is the purpose of the black gems in Gem Spinner II?
They are there as a challenge. They cannot be removed with a match, as other gems can. Three black gems in a row doesn't make a match. However, you can get rid of a black gem by having it inside of
a piece at the time that piece is removed from the board.