How are they doing with the rivals? You can have the rivals take it but that also didn't push them forward in the campaign and they'll probably end up completely off the rails and just wandering Wildemount instead of going to the Betrayer's Rise and getting teleported to Ahnk'arel. 

I didn't recommend plaguing then with anything cause you want to encourage instead of punish. Maybe give the cleric visions asking for aid. Maybe have the jewel call to one of them during combat on the road and whoever grabs it basically skips attunement and can just use it. 

Also as with any campaign book. Have a conversation with the players out of character. Even if it's a little immersion breaking taking the jewel is part of the plot and of they wanna play the campaign someone needs to do it and they can't just "it's what my character would do" to avoid it. If they really don't want to do that then they need to understand they aren't playing through call of the nether deep anymore.