What does Igor say about the ring, or the platform, and did you talk to him a second time?
Did you try talking to Sassan and see if you can upgrade the necromancer platform? If you didn't finish the upgrade for the first quest, that could be why you are not getting the second quest. Alternately, if you did, maybe the second quest is already completed and you just need to talk to Sassan for the upgrade.
I've killed quest related NPCs and looted associated quest items before getting the relevant quest, without any problems. I didn't pick Igor in D2:ED or DKS, though.
Are you playing the original release of D2:ED, or DKS / D2:DC? With D2:DC, the Developer Mode can be used to give the character equipment, which may include Stone's Ring. Someone did find a way to
activate the developer console in DKS, though I don't know if it could give specific equipment (AFAIK DKS saves are compatible with the D2:DC release, so it would be easier to get someone to try the official developer console).
