If you have wide cheekbones and tapered forehead and jawline then you most likely have a Diamond face shape. However, if your forehead and hairline are wider than your cheeks and lower half, then you're a Heart face shape.
Be Open to a Secondary (or even a third) Face Shape
You may not be an exact Diamond so you probably have a secondary facial shape. It depends on your features. If you pay attention to the proportions and bone structure of your face, you'll know what you need to work on to visually widening your forehead/hairline and chin area.
But if you have a wide face (like for example Linda Evangelista and Audrey Tautou who have a shorter forehead and broader cheekbones), your focus should be on slimming your overall face shape and follow the guidelines for Round Face Shape. Or if you have a strong jawline see my tips for Triangular face shapes.
Many Diamonds also have Oblong-tendencies. So if your face is narrow along with a long and pointy chin (like Jennifer Love Hewitt's and Meg Ryan) you might want to shorten the overall length by following the tips for Oblong.