No, they have to be stone. The network is defined by the sign, not the blocks the portal is made out of. In fact, if you made the portal out of anything but stone, it wouldnt work (not my rule, just how the plugin is written). I changed it to stone from obsidian because its easier for people to get stone than obsidian (only donators have access to /gamemode and /i, and obsidian is a BITCH to collect.)