In this picture the filler is clearly visible on the stone's surface. This is due to the

 difference in the refractive index of the glass filler and the ruby. The darker patches

are easiest to see when the stone is examined in a reflective light. Polishing

lines are also clearly visible.

Two gas bubbles clearly visible in a lead-glass filled ruby. This is a fairly

 common inclusion in these stones.

The lead-glass filling stands out in high relief in this lead-glass filled ruby.

Many lead-glass filled rubies show interference colours when the stones

 are examined under a microscope. The colours are best seen using a

 fibre optic light but are also visible using dark field illumination.

The lead-glass filling stands out in high relief in this lead-glass filled ruby.

The lead-glass filling stands out in high relief in this lead-glass filled ruby.

The lead-glass filling stands out in high relief in this lead-glass filled ruby.