The Masu are literally the "Base of All."

That does not mean that they are actually the basis of
the universe.  They are just a simple, gelatinous
lifeform.  Their name is derived from their
exceptionally basic nature. The Masu are just above
basic quintessence, but are stuck in a state of
permanent "goopiness." They can take any path of
development possible under ordinary physical rules,
but like Hamlet, they choose "not to be", hence just
definitionless sludge.

But their reactivity in the presence of quintessence
is extraordinary.  Ordinary matter, in alchemy, can
only attain one of three states: It's current state,
its quintessential state, or a final transformative
state. In other words you have lead; lead is
transformed by quintessence into a transitory phase;
the transition becomes gold.

But the Masu can be turned into anything, instantly. 
And back again, with a minimum of effort.   The Masu
can be driven from their base state, with the proper
direction (human judgement), and the energy of the
gems (most preferably Gem Theory 2 or 3), the Masu may
be ALL and Everything, since they are limited only by
the current quintessence rules (the rules of matter
and energy in a given universe). 

The actual properties of the Masu are simply energy
release and mass accumulation.  But such simplicity
makes them utterly pliable.  The introduction of
complex symmetries, and more advanced physical or
chemical rules can turn them from goop into complex
machinery or lifeforms.  Of course those symmetries
are quintessential, which is why the interplay of
Ryoko and her gems are so important to the task at
hand. Concievably, there is nothing that cannot be
done to the Masu, as long as their is conformance to
quintessence.

You can even replicate the One.

Peter 
-Hey Rep, I do turn off the computer and go read a
book. That's how this all got started.  Don't you know
that the best weapon in the Revolution is a library
card?