Hi Anudippal;
Fracture filling hides feathers that reach the surface, making them clear. An I 2 diamond can become an SI 2... But it STILL is I 2. Basically the look is enhaced, but the stone is weaker than a natural SI 2. The subtance used to fill the feathers does not add weight. You can recognise a fracture filled diamond because of its 'flash' effct. Sometimes a stone is drilled to reach a prominent feather, shouldn't it reach the surface. The most important lab that grades fracture filled diamonds is EGL. I suggest you to have a look at yehuda.com, which is one of the most important enhacement companies.
Laser Drilling maked dark inclusions look clear. It's permanent and isn't reversible. It's more recognized than fracture filling and a stone with such treatment will be graded by almost all of the most important labs. Sometimes the drill is filled; in this case some labs won't grade them and such diamond should be considered drilled and filled.