The recipe for Quartz slivers was broken in the 5.10. I was able to make them fine in 5.9 and the recipe shows up fine in JEI but it just doesn't work.

Have you tried updating to 5.10.1? There was issue regarding crafting recipes

It is working for me. Have you changed anything? Or, have you double checked that it is 5.10.1? I think someone mentioned an issue from twitch that it still gave them 5.10.0 for some reason. I think that issue is taken care of, but something to check.

Sorry, should have been more explicit as we're running 3.5.10.1 already.

I double checked and it's still 5.10.0 even though Twitch is showing 5.10.1. Going to downgrade/upgrade and see if that resolves it.

Resolved by downgrading/upgrading and upgrading server