Time-worn Black Stones no longer exist (Bonus info: they used to be special seasonal server items to enhance Tuvala gear).

Question for you: Are you trying to enhance a TET Giath's Helmet that you got from Jetina for exchanging a PEN Tuvala piece? And is it showing you a greyed-out Time-worn Black Stone in the enhancement material slot? If so, that's a (misleading) placeholder.

You cannot actually enhance TET Jetina gear pieces normally. Instead, you have to craft 5 special Reform Stones and use your black spirit's Reform option to upgrade your helmet with them. I recommend looking up a guide on this specifically, as it's a bit in depth, but IMPORTANT NOTE: the total cost of crafting the Reform Stones is very expensive. You're given a good chunk of the necessary materials for your first stone as a new player, but I recommend tallying up the material cost and comparing with the Central Market price of simply buying the PEN boss piece before committing to the Reform Stone path. When I did it a couple weeks ago it was almost even.

One small trick that is not in many guides: For the 5th reform stone you have to melt down boss gears (either one TRI gear, two DUO gears, or three PRI gears). Any boss grade gear works for this, including boss weapons. If you search through all of the different class boss weapons on the market, you may be able to find a random TRI weapon that someone is selling for way less than a TRI armor piece.

Businessox is spot on! Here is a helpful guide from BDOFoundry as well about this process.