Some diamond they have imperfect, many fracture and cracks on it and the fracture of course detract the appearance itself and hence affected the value of the diamond.
This process produced by Israel and the United States begun in early year 1980's which the technique was developed for filling the fracture and with a glass material.
The glass filler with high refractive index which close to the diamond makes the fracture become less visible. And it has been believed that this treatment performed under by heating and pressure to fill the liquid glass into the fracture.
This process is not an Ultrasonic cleaner, high temperature heating by jewelry repair and avoid an acid is a must. If not in the most cases the liquid will evaporate and leaving all cracks become more visible.