Stars and flowers work together because they both feel whimsical and fun. The combo is also a great subtle way to try the floral trend, if a full-on print dress isn't quite your thing. And, yes, you can mix gold with silver!
The chains toughen up the jewels, making them less big-evening-out dressy—what would be a night look turns into something perfect for day.
I contrasted stark, clear gems with black onyx, though you can also match brights with brights, or mix different tones of the same shade...any complementary color combo will do. Just make sure that the necklaces you use are of varying lengths.
You don't have to choose between your favorite pendant and that new necklace you just bought: Wear them at the same time!