I haven’t checked, but I’d be very surprised if it were stereo.
There are two jacks on this headset. One is a mono mic and the other is a stereo speaker. It’s a Logitech headset made specifically for computer use.
Ok now I’m upset and I feel foolish.
There are indeed two jacks: one for the cans and one for the mic. But you are all right…the mic jack is a stereo one. And it gets better: the adapter I’m using is the one for my cans (as I noted) but I didn’t make the connection (pun intended) that the adapter too is stereo. Ugh.
I feel stupid. 
problem solved then?
I’m sure we have all done something similar . 
Here , a little British humour to cheer you up ;
Not until I sneak down to Wal-Mart or Staples and buy an stereo to mono, 1/8" to 1/4" adapter. (Do such beasts exist? Time for Google!)
Edit: the closest Radio Shack has it. But it’s considerably farther away than Staples.
It didn’t work. Meh. I give up for real this time.
If it was me I’d cut the stupid 1/8" plug off and solder on a proper plug, like an XLR, you’d also see if the mic was running a balanced connection or not.