  We exploit James Webb Space Telescope (JWST) NIRCam observations from the
GLASS-JWST-Early Release Science program to investigate galaxy stellar masses
at z>7. We first show that JWST observations reduce the uncertainties on the
stellar mass by a factor of at least 5-10, when compared with the highest
quality data sets available to date. We then study the UV mass-to-light ratio,
finding that galaxies exhibit a two orders of magnitude range of M/L_UV values
for a given luminosity, indicative of a broad variety of physical conditions
and star formation histories. As a consequence, previous estimates of the
cosmic star stellar mass density - based on an average correlation between UV
luminosity and stellar mass - can be biased by as much as a factor of ~6. Our
first exploration demonstrates that JWST represents a new era in our
understanding of stellar masses at z>7, and therefore of the growth of galaxies
prior to cosmic reionization.
