Find your girlfriend's pinterest. OR look at the style of jewelry she already wears. Alternatively, and I think this is a great idea - Buy a plain, cubic zirconia 'pop the question' ring and then shop for a ring together?
Popular alternatives to
maikun Womens 18k White Gol...
FlameReflection Black Stainless Steel Princess Cut Simulated Amethyst Cubic Zirconia Criss Cross Rings Wedding Engagement Ring for Women Crossover Design