"A colorless, marquise-cut gem."
— Item examination
This treasure can only be found during the Volcano segment of Chapter 6-3, and only if playing as Chris Redfield.
After landing in the volcano, look along the path; just beyond the discolored rocks that will give way under Chris is a short tunnel created by a pillar that arches over a pathway. Firing any gun at the lower segment of the pillar will cause it to crumble into the lava beneath and create a stepping stone.
When Chris's bridge falls after escaping from Wesker, Chris can jump over the stepping stone to a small island. The Marquise Diamond is on that island.
- kaitaishinsho, p.445.