He found what he wanted. Perfect conduit and amplifier for his powers, as well as storage unit. He had to deal with the technical issue of leakage. Souls were a fairly solid and unified spiritual element, and drawing power from them, althoug Gavin's natural talent, was not easy. They either had to be with the living owner or on the 'other side'. And while the purity of the crystal was enabled it to retain the energy, it was inevitable that they would eventually slip out.
Gavin could only affect how long would it take for the quartz to retain this energy. The notes mentioned soulstones, but those were semi-legendary and their creation was a difficult and arcane process, combining alchemy, high-level magic and near-instinctual understanding of the nature of creation. Gavin was on the opposite spectrum of resarchers, going through brute force experiments and re-engineering tested formulas.
It was then the matter of streching the energy retention from hours to years to make it a viable item, preferrably centuries if possible. Maybe the stone was too rough and had too big of a surface area? Maybe he had to follow some specific design? Or maybe the stone had to be transmuted?
All three options combined seemed like a step in the right direction. First design, then precision cutting and polishing the stone and finally a ritual to imbue the stone with part of Gavin's own soul-manipulating ability, to slow down the energy leaking to naught and magnify its already impressive magical properties. He would need a considerable amount energy to make the artifact powerful though, and Cain was no William. He had to plan and round up people that would not be needed by anyone. Slaves, criminals, beggars. Gavin abandoned any moral considerations long ago, and the prospect of arcane power was worth sacrificing few hundred useless people.
Research for appropriate stone design, then thave it cut and polished to design. Request meeting with Cain.