I use diamonds with a "Round Brilliant" cut, which has 58 facets. The top of the stone is called the crown and the flat at the top of the crown is called the table. The bottom of the stone is called the pavilion. The girdle is the largest diameter of the stone and is the part that is clamped in the mount. If the stone is properly cut, light enters the diamond through the crown and is totally reflected from the facets in the pavilion and back up throught the crown. This is the principle of "total internal reflection". It is not necessary for light to be transmitted through the pavilion for a stone to "shine" brilliantly. Incident light from the top is generally sufficient.
Another thought: for a diamond to perform well, it must be kept clean. Buildup of grease, dirt, funk, whatever - should be thoroughly cleaned off. Junk and trash on the pavilion is also detrimental, since it can eliminate the "total internal reflection" principle.