Free text search is not part of the SPARQL specification. Some products, such as ARQ (part of Jena) and Virtuoso, do have bespoke implementations.
The extension to SPARQL provided by Virtuoso is documented at:
Virtuoso and free text search
An example of its use that will return a non-empty set of results from
SPQR’s Virtuoso installation is:
PREFIX spqr: <http://spqr.epcc.ed.ac.uk/>
SELECT distinct ?material
?p spqr:material ?material
FILTER bif:contains(?material, "MARBLE")
Why not try it out on SPQR’s SPARQL Endpoint?