  Here we report the systematic study of solid-state phase transformations
between boron polymorphs: {\alpha}-B12, {\beta}-B106, {\gamma}-B28, T-B52 and
amorphous boron (am-B). It is evident that the Ostwald rule of stages plays an
important role during phase transformations not only of amorphous boron, but
also of crystalline forms. We have observed the crystallization of tetragonal
boron T-B52 from amorphous phase of high purity (99.99%), which, however,
cannot be easily distinguished from B50C2 boron compound. Many factors
influence the transformations of amorphous phase, and it is possible to observe
not only well known am-B \rightarrow {\alpha}-B12 and am-B \rightarrow
{\beta}-B106 transformations, but also am-B \rightarrow T-B52, never reported
so far. At ~14 GPa the crystallization order becomes {\beta}-B106 \rightarrow
{\alpha}-B12 \rightarrow {\gamma}-B28, while at ~11 GPa the intermediate
crystallization of T-B52 still was observed. This unambiguously indicates that
{\alpha}-B12 is more thermodynamically stable than {\beta}-B106 at high
pressures, and renders possible to transform, at least partially, common
{\beta}-phase of high purity into {\alpha}-B12 at very high pressures and
moderate temperatures (below 1600 K), i.e. outside the domain of its stability.
