Okay, I've gone over it for grammar/spelling as well.

Here's what I found.
Appearance:
When they’re created, They begin in a “seed-core”
Lowercase the 'they'.
Mature Lavak:
Somewhere in the lava floats a large crystal that grows bigger the longer a Lavak survives until it shatters as the Lavak dies.
This is just kind of awkward. Maybe something like this:
Somewhere in the lava floats a large crystal. This crystal grows for as long as the Lavak lives, and shatters upon the Lavak's death.
Abilities:
If one didn’t know better, he wouldn’t even imagine that the seed-core and Mature forms of the Lavak were the same creature.
Lowercase the M.
Seed-Core:
thus it’s name, “Lavak Powder.”
its, not it's.
Anybody with a good-sized sledge hammer could break one of these, though sharper implements are more likely to just glance off of its' body.
sledgehammer, no spaces.
'although' sounds better than 'though'.
its, not its'.
Mature Lavak:
It’s body is composed entirely of lava surrounded by a thin membrane
Its, not it's.
it would be able to shape itself into any shape fathomable and light fires by its touch
Just awkward. Maybe 'and set things on fire by touching them.'
because of the pressure differential.
'because of the difference in pressures' works better than 'pressure differential'.
Personalities:
Those instincts are primarily guided towards survival although Ivak did imprint a few that would further his purposes.
survival
, although... Put a comma in there.
Lifespan:
Where a shard of crystal lands in a puddle of molten goop, a seed-core Lavak is formed.
When a shard crystal lands in a puddle of molten lava, a seed-core Lavak is formed.
Where=when, and goop=lava.
Some few are even harvested by people looking to use their body to make explosives.
This sentence seems a little vague... almost like you're inferring the humans are going to use their bodies. The few seems out of place. Maybe:
Some are even harvested by people looking to use the body of the seed-core to make explosives.
When the seed-core finally locates a heat source strong enough to light it on fire, it will burst into a huge explosion, the force of which has been known to kill bears twenty feet away.
This is awkward. Maybe something like this:
When the seed-core finally locates a heat source strong enough to ignite it, the seed-core will explode. The force of this eruption has been known to kill bears twenty feet away.
While mature, the Lavak act as a group to relieve the pressure of the underground whenever it grows too high in their particular sector of the underground.
Underground twice, seems redundant. The while seems to be set to pose an 'as if' thing, which isn't your intention. Maybe something like this:
Mature Lavak act in concert in order to relieve the underground pressure when the levels are too high in their territory.
seem to be of negligible benefit to humanity,
There's other races besides humans in the area. Maybe 'those living on the surface' instead.
When the Lavak reaches the surface, it explodes and starts the process from which it originally came all over again.
explodes, and. Put a comma in there.
Wthen the Lavak reaches the surface, it explodes, and begins the process from which it originally began once more.
Otherwise, I'd say it will be good to go.
