This is the technical term used for heat treated gems stone. The low graded stones which are opaque and are not bright are treated with chemical like acid first which makes the stone brittle and porous. Then these stones along with silica and other chemicals are heated.

The pours of the stones are filled with the silica at high temperature and thus giving a transperancy and brightness to the stones. These stones are called fissure filling astrological stones.

If any ordinary person will go to any jewelry store and ask for such stones, they will not get and more over stone seller don't tell his customers that the stones are heat treated.

The results of the stones is due to its refractive index of that stone. In fissure filling stone the pores of the stones are filled with the chemical like silica under high temperature to create transparency and brightness. Due to additions of the chemical and its fusion at high temperature the property of the stones changes and so does the refractive index of the stones. This is the reason that this stone will not exactly catch the wave length of the planet they represent and thus will not give results.

The refractive index of the original stone changes with such treatment and so will be its effects ( guns ) so its always better to avoid such stones.