The chrysanthemum stone is approximately 270 million years old. It is a dark gray to black limestone matrix with andalusite crystals taking on the appearance of a chrysanthemum and contains more than 10 kinds of trace elements, such as selenium, stronium, gold, silver, & bismuth.
The black rock itself is a piece of limestone. Although limestones generally are lighter in color, this one is quite black because of the presence of substantial amounts of organic carbon. This rock is Permian in age, from a time when many areas that today make up China were offshore or underwater plateaus.
The white pattern on the rock is formed of crystals of celestine (or celestite), a strontium-sulfate mineral. Celestine commonly forms elongate crystals of the form seen here, although these are fairly rare in that they’re so thin and on the surface of another rock.
Strontium is often concentrated in carbonate rocks and probably grew on these larger limestone surfaces as a consequence of flowing water after the rocks were deposited; a small crystal could nucleate at the center of the flower and crystals would then grow outwards from that center point.
Chrysanthemum Stone was formed from geological changes in the Permian age of the Paleozoic era, 248 to 290 million years ago, from high temperatures and the compression of thick layers of organic-rich mud at the bottom of the sea. As strontium sulphate (Celestite) and calcium carbonate (Calcite) oozed into the mix, continued pressure and deposition allowed spectacular radial lath-like crystals to form that remained as flower patterns.
These “flower” crystals consist of Celestite, Calcite, Feldspar or Andalusite, while the black, or sometimes brown, base rock of Chrysanthemum Stone is comprised of Dolomite, Gypsum clay, Limestone or Porphyry.
The stone may naturally expose the flower, though marketed specimens are often highlighted by selective painting of the black matrix with a dye to accentuate the pattern, or it may be carved away or sandblasted for emphasis.
Source – Geologyin