When you purchase a diamond from reputable diamond ring stores, the chances for you to get scammed is almost zero. They will give you an authenticity certificate along with your diamond as well. However, things are not the same in the case of pawnshops. In most cases, they sell cheap diamond rings that do not carry an authenticity certificate. Hence, the chances for you to get scammed or ripped off here are more.
You should thoroughly check and evaluate the diamond engagement rings on sale well before making a purchase. For this, you may take an appraiser or gemologist along with you. Otherwise, check for the cut quality, color, clarity, and shape of the diamonds. Note that cut refers to the proportions, facets, angles, and other finishing details of the gemstone, and not the outline of the diamond as most people mistake it for.
Besides that, also make sure that the gemstone is eye-clean. That is, it must not have any visible blemishes and appears colorless when viewed with your naked eyes.