  It is highly desirable for future measurements of Newton's gravitational
constant $G$ to use test/source masses that allow nondestructive, quantitative
internal density gradient measurements. High density optically transparent
materials are ideally suited for this purpose since their density gradient can
be measured with laser interferometry, and they allow in-situ optical metrology
methods for the critical distance measurements often needed in a $G$ apparatus.
We present an upper bound on possible internal density gradients in lead
tungstate (PbWO$_4$) crystals determined using a laser interferometer. We
placed an upper bound on the fractional atomic density gradient in two PbWO$_4$
test crystals of ${1 \over \rho}{d\rho \over dx}<2.1 \times 10^{-8}$ cm$^{-1}$.
This value is more than two orders of magnitude smaller than what is required
for $G$ measurements. They are also consistent with but more sensitive than a
recently reported measurements of the same samples, using neutron
interferometry. These results indicate that PbWO$_4$ crystals are well suited
to be used as test masses in $G$ experiments. Future measurements of internal
density gradients of test masses used for measurements of $G$ can now be
conducted non-destructively for a wide range of possible test masses.
