Need material for gramp

Only if you absolutely need to level him up. I'd recommend about half of each apple to use and keep the rest for later events if you decide to just slam those apples in.

Depends on how many of an item you need. If it's only a few, then a few apples MAY be worth it. But if you need a lot, then it may not be worth it given the horrible drop rates.

I needed the Gallstone materials a while back, just 5 of them. Without using apples, it took me nearly a week's worth of natural AP to get them.

Personally, the only materials I farm are skill gems. For anything else, I wait for events unless I only need a few of them. Even then, I don't use apples, even when I had like 150+ golden ones. Apples would be better off used for events, especially roulette events like Nero Fest where you can farm an unlimited number of the available materials.

Cursed beast gallstone is almost unfarmeable, My. Ebin drop it in worse rates than hearts in Caster Dailies, wait for event shops.

Chains and Needless are easy to farm, but if you need like dozens of them, is better to just wait for event shops.

Ghost Lantern drop rates in Storm Seas is like 30%, if you need a lot, dedicate to events. If not, daily farm with natural AP. Expect to lost a lot of time to get more than like 3...also, Dantes event sure-drop a few...

Yellow Gems, oh, Yellow Gems...Xuangzang event rerun will have gems in the shop, NeroFest also has gems in the shop and Christmass has in the roulette. You can do 40 AP Assassin Item Farm 3 times every week and pray for the drop rate to be generous...

Go for the ascension mats first and save the 10/10/10 requirements for the upcoming events. Save yourself some sanity...

I managed to grind one Cursed Beast Gallstone that I was missing, but a total of six would be hard. You need to run the recommended quest 8-9 times to get one drop on average.