This video discusses how to avoid shadow lines in your seams.
To eliminate shadow lines, the goal is to get your adhesive to have the same opacity or translucency as the stone you are working with so that the light passes through cleanly.
If the adhesive is opaque and the material is translucent, you will see a grey line where the light hits the opaque seam and bounces back out.
If your material is opaque and the adhesive is translucent the light will go into the seam and bounce around between the two pieces of material creating a shadow line.