How to use Crystal Ball/Fire Heart?

Hi,
I've heard a lot of good things about Crystal Ball on this forum (supposedly it's one of the best spellcasting items?) and Fire Heart looks really good on paper (just think of how many potions it's equal to) but I have some questions.
When are you supposed to use Crystal Ball? If I use it for leveling I can't afford to use it when I get to the boss (the gold cost is too high) but if I only use it for the boss, I'm passing up on items that could be useful for both leveling and the boss (Dragon Soul, Battlemage Ring, etc). Also, when I'm trying to use it to spike the boss, I can rarely get more than one or two refills from it because once you use it once, it's not worth using again until about another 12 glyph casts (it's rare to get such a big mana spike). Finally, the price for Crystal Ball just seems too high to me? For the same amount of money, I could have bought an item that equally helps my spellcasting, and also a potion on top of that, for instance. On dungeons that aren't generous with gold, that's pretty important.
As for Fire Heart, I can see the point of it more since it doesn't have an innate gold cost. Should I just be using it when it's at 90% or higher whenever possible to spike tougher enemies during leveling? Or is it more worth it to use at only about 40%-50% restoration since that will charge faster?
I've heard a lot of good things about Crystal Ball on this forum (supposedly it's one of the best spellcasting items?) and Fire Heart looks really good on paper (just think of how many potions it's equal to) but I have some questions.
When are you supposed to use Crystal Ball? If I use it for leveling I can't afford to use it when I get to the boss (the gold cost is too high) but if I only use it for the boss, I'm passing up on items that could be useful for both leveling and the boss (Dragon Soul, Battlemage Ring, etc). Also, when I'm trying to use it to spike the boss, I can rarely get more than one or two refills from it because once you use it once, it's not worth using again until about another 12 glyph casts (it's rare to get such a big mana spike). Finally, the price for Crystal Ball just seems too high to me? For the same amount of money, I could have bought an item that equally helps my spellcasting, and also a potion on top of that, for instance. On dungeons that aren't generous with gold, that's pretty important.
As for Fire Heart, I can see the point of it more since it doesn't have an innate gold cost. Should I just be using it when it's at 90% or higher whenever possible to spike tougher enemies during leveling? Or is it more worth it to use at only about 40%-50% restoration since that will charge faster?