[quote=@Bishop]
So, human children can be mutated by Lapi usage but does that also happen with grown ups?

And everyone can use Lapi but is there a limit to how much the crystal can be used consequently without stop?

 Does it overheat, discharge then needs time to recharge or wear down the users own energy? 

By what I read I'm guessing that a Lapi has the same effects no matter who uses them but when you said that master magis could manipulate it, do you mean that instead of a, for example, lightning bolt they can manipulate it to take the shape of an arrow or a lightning sword?

 If there are multiple lightning Lapis with different effects, if someone masters one of them and learns to manipulate it, does it mean that he can virtuality be able to fabricate all lightning Lapis just from mastering and manipulating one? On a side note can you manipulate a Lapi without actually mastering it? So the magi can't recreate that Lapi 's effect by himself, he needs one to pull it off, but at the same time he has learned to manipulate said Lapi. So if he had a lightning spear Lapi, now he can manipulate it and change it to a lightning sword or lightning bolt. Now that Lapi for him* is a lightning Lapi with universal use, meaning that he can recreate all lightning Lapis from that single one he had. 

What are the benefits of mastering a Lapi instead of just carrying it and using it besides  the fact that you don't have to carry it on you all the time? Does the casting procedure change? Does it require more of your inner energy to be spent?

And finally, could you mold Lapis in your body? I mean if you molded a fire/water/earth/air on your body and learned to manipulate them all, you practically become the avatar.
[/quote]
I'll separate your question into parts.
1. Human children aren't mutated by using Lapi themselves, they're mutated by people using Lapi around the mother while pregnant. This means that adults can't obtain mutations either.

2. Generally, Lapi cost stamina. Use Lapi too much, and you'll pass out. Use them [i]too[/i] too much, and you can die of exhaustion.

3. Occasionally they have cooldown periods where they can't be used, but they will never overheat. And yes, it does wear down the uer's energy.

4. Correct.

5. Yes, they can mimic other lightning based Lapi, but without mastering a Lapi, it's incredibly dangerous to attempt to manipulate an unmastered Lapis, and will backfire at least 99% of the time. But yes, if you have mastered a Lightning Lapis you can recreate all lightning based Lapi, at the cost of much higher stamina use.

6. The benefits of Mastering Lapi are listed thusly; You don't need to carry it around, you can manipulate it, and it costs less stamina. The casting procedure is kind of different, at least in style. Instead of pouring your energy into the Lapi, you pour it into your soul. But that's just fluff. You cast it pretty much the same, charge something with your stamina, make magic happen.

7. By mold I assume you mean master. Yes, if one did master a fire, earth, water, and air Lapi they could become the avatar. Unfortunately, that would take a lifetime of work from someone with good magical potential, like a human. That is a good idea for a folk hero though... expect to hear a bard singing about that whenever the group strolls into town actually.