Illithium Alloy is a hybridized metal, a melding of illithium ore and some unknown other substance that Jerlia can create for Gorion's Ward and their party in addition to, or instead of, seeking out Unger Hilldark.
Illithium alloy takes the form of a rough trapezoid of shining silver material. Although indistinguishable to the common eye, to a trained metalworker, it is clearly not pure ilithium, as it holds smears and smudges.
If presented to Sir Sarles, he will be fooled for only a moment, then realize what it is and, insulted, he will leave Athkatla. Upon reporting their failure to the sub-priest, the high priest will spot the alloy and mistake it for Sir Sarles' sculpture, happily accepting it as a worthy dedication to their god.