Creation of magic energy in an attempt to make a somewhat authentic word based magic system

[Wolk]

Greetings,

I am currently attempting to construct a somewhat authentic word based magic system for a potential (medieval high fantasy) writing setting and am quite happy with what I have achieved thus far.
Here are some articles from my MediaWiki which provide necessary information:
Magic and Magicka

However, I struggle to find a satisfying solution to the creation of magicka, as I don't want it to just "plop" into existence. Some of my thoughts were:

• A crystal that converts heat energy into magicka.
  • Problems:
    1. How?
    2. Potential earth-cooling.
• Several things which convert chemical energy, light, sound, wind or kinetic energy into magicka.

These ideas seem to be the most promising of solutions, though for the second point I am not sure what would transform the listed energies. Some sort of magical tree seems most likely for chemical energy and light. Then I am also worried about the finiteness of these as an energy source, as magic is supposed to be god-given, but it is something I concider.