I think a border of alternating black with your jewel colors would look nice. Each block would be half a triangle. Use your blue strips (for example), and only make the bottom half of your strippy triangle. It is a 4-sided figure, a flat top with sloping sides. Try several different varieties of borders, and audition them, before committing to one. Love double and triple borders, adds a look of strength and stability to scrappy quilts, in my view.