When shattering objects with transparent materials, it is necessary to control the refraction for each individual piece. Otherwise the object appears already shattered, as refraction will make the borders of the pieces visible. In this scene is a simple setup which controls the "Visible by Refraction" option of Compositing Tags. You can set the minimum force necessary to enable the refraction for the piece.
Note: Two shortcomings of the current version:
Once a piece falls again below the given threshold, it will loose the refraction
The Shadow is not affected by this setup, so borders will be visible