Diamonds with imperfections, e.g. cracks or black pinpoints, are less valuable than those without.
Some manufacturers have worked to give nature a helping hand by using new techniques to improve the appearance of natural (but imperfect) diamonds.
These techniques include fracture filling, irradiation, and external coatings.
All of these treatments are completely legal, as long as they are disclosed to the buyer and are not meant to misrepresent the product. Any such misrepresentation is illegal.
Fracture filling is a semi-permanent process in which a foreign substance is infused into cracks in the stone to improve its naked-eye visual clarity.
Because the filling is not permanent, GIA GTL and most other gem labs will not grade fracture-filled stones.