Dilithium

Dilithium is a naturally-occurring material that forms a vital part of almost any antimatter reactor.
Any atom of Dilithium is a Carbon-12 atom, but with half its subatomic particles displaced into subspace, making it appear to be an atom of Lithium-6, an isotope of Lithium. It would behave chemically and physically like Lithium. However, its semidisplacement causes it to respond slightly differently to certain types of scans to "real" lithium, namely parametric subspace field stress sensing and sub-baryonic thoron spectrometry, which allows Dilithium to be identified.
When bombarded with correctly-phased fast neutron radiation, the displacement of the subatomic particles can be "balanced" out, resulting in an atom resembling Carbon-12, but which is very slightly out of phase, rendering it resistant to antimatter that is not similarly phased. Because Carbon forms crystalline structures, Dilithium correctly refined can form crystals similar to diamond, though a very small minority of atoms will undergo induced fission and become phased boron, giving the crystals a purple hue. Dilithium becomes partially porous to superheated matter and antimatter when exposed to exahertz EM radiation in the megawatt range; its displacement also suppresses annihilation reactions inside it, allowing matter and antimatter to fully mix into a collimated stream which annihilates upon leaving the crystal. 
Dilithium crystals are dephased by exposure to matter and antimatter, though this is incredibly slow; the danger here is caused by individual node atoms being dephased and annihilated by antimatter, which causes the crystals to fragment over time.
Dilithium ore might look like this lump of Ambligonite, a Lithium ore, but it can take many natural forms, none of which resemble the purple crystals of the material's refined state.
Regarding the briefly-mentioned refinement, there is quite a balance to be struck here; too much radiation and the atoms will be entirely dephased and become nothing more than lumps of radioactive coal, but too little will cause a buildup of subspace stress, which will be released at a practically unpredictable time as a subspace displacement burst strong enough to tear apart primary structural members of a vessel or starbase, not to mention the bones and organs of those within.
Due to its phased nature, dilithium cannot be replicated, and it cannot be manufactured either, as the mechanism by which it is produced is unknown and appears to have halted in this galaxy long ago. As such, dilithium crystals are sometimes used as a medium of exchange between spacefaring polities.
Category:Lore