  Sulfur is one of the most abundant elements in the Universe, with important
roles in astro-, geo-, and biochemistry. Its main reservoirs in planet-forming
disks have previously eluded detection: gaseous molecules only account for
$<1\,$\% of total elemental sulfur, with the rest likely in either ices or
refractory minerals. Mechanisms such as giant planets can filter out dust from
gas accreting onto disk-hosting stars. For stars above 1.4 solar masses, this
leaves a chemical signature on the stellar photosphere that can be used to
determine the fraction of each element that is locked in dust. Here, we present
an application of this method to sulfur, zinc, and sodium. We analyse the
accretion-contaminated photospheres of a sample of young stars and find
$(89\pm8)\,$\% of elemental sulfur is in refractory form in their disks. The
main carrier is much more refractory than water ice, consistent with sulfide
minerals such as FeS.
