Re: いせたん [RJ130644]
"Magic stone" is probably a more fitting term. Most of the world's technology run on it lorewise.
As has been said in the past, the bag is reobtained after the Temple Knight Siege arc, from an NPC thief that shows up in the slums; it's a LONG way there, and by the time you get there you most likely have way better accessories already. +20% resistance to more or less everything can help, admittedly.
The clock, on the other hand, takes a few errands - first one needs to hand it over to Luigi (blacksmith guy in the castle town, green hair), earliest this is possible being the free-roam before the Fairy Forest/Undead Invasion arc, and he'll then hand it over mid-way through the Temple Knight Siege arc. May need to do the steps leading up to that point correctly (finishing the Graveyard Dimension first, grinding up the Miko class, etc). Pretty much everything it can do the player can do better with other spells/skills already, but that lv1 damage null barrier can be useful.
Also, gaining levels in the cave is... Somewhat irrelevant, since right afterward you'll most likely want to grind the rats in the labyrinth entrance area while Iruda is in the party to get the Adventurer Apprentice job class leveled to a decent point.
EDIT: if you mean repairing your broken items, then all blacksmiths offer that service, or you can use the blue cubes or hourglasses the guild guy (same NPC you sell stuff to) - among others - sells to do so. Note that repairing actually broken items (0 durability) require the more expensive 500/5000 items; the 250/2500 ones can't.