  Heralded generation and manipulation of quantum entanglement between two
macroscopic and spatially separated crystals at room temperature is
theoretically studied. We show that by combining an x-ray parametric
down-conversion source and x-ray interferometry with nuclear resonant
scattering techniques, two macroscopic crystals hosting M\"ossbauer nuclei
located each on an interferometer arm can be entangled for few tens of
nanoseconds. The coherence time of the entanglement state can be prolonged up
to values comparable to the lifetime of a single nuclear excited state, on the
order of hundred nanoseconds. A non-mechanical magnetic control of the quantum
phase between the two spatially separated entangled nuclear crystals is put
forward.
