What ever method of repair you use, short of having the stone recut, be
careful of the cleavage planes.
Topaz has a perfect cleavage plane. It’s possible to apply force paralle
l to the cleavage plane & split the stone into 2 pieces. Usually topaz
is cut with the cleavage plain 10-12 degrees off the table.
I don’t know of any way to look at a cut stone & determine the location of the
cleavage plane. Depending on the appearence of the chipped area it
ma y be possible.