So-called because when you crack them open, their internal cross-sections look like tiny golden-pink flowers, cherry blossom stones (sakura ishi in Japanese) get their beautiful patterns from mica, which is a commonly found silicate mineral known for its shiny, light-reflecting surface.
These flower patterns weren’t always made of mica. They started their existence as a complex matrix of six prism-shaped crystal deposits of a magnesium-iron-aluminium composite called cordierite, radiating out from a single dumbbell-shaped crystal made from a magnesium-aluminium-silicate composite called indialite in the centre.
Hosted inside a fine-grained type of rock called a hornfels - formed underground around 100 million years ago by the intense heat of molten lava - cherry blossom stones underwent a second significant metamorphosis in their geological lifespan when they were exposed to a type of hot water called hydrothermal fluids. These fluids altered the chemical composition of minerals inside the cherry blossom stones, causing mica to replace the original cordierite-indialite inclusion.
Because they have to undergo two intense and very specific types of metamorphosis in order to form, cherry blossom stones are incredibly rare, and found - rather serendipitously - only in central Japan.
Not all cherry blossom stones experienced a complete replacement of their internal minerals during their geological lifetime, but those that did are quite delicate inside, according to a 2006 study published by the journal Rocks & Minerals by John Rakovan from the Department of Geology at Miami University in the US. Rakovan reports:
"They can easily be snapped in half or crushed between one’s fingers. Although they are delicate, complete crystals, showing well-preserved external morphology, are commonly found weathered out of the hornfels. In areas where the cordierite is completely replaced by mica the hornfels is also altered such that it is very friable and poorly consolidated.”
In order to preserve the beauty of their delicate mica patterns, the Japanese locals coat them in a diluted solution of wood glue mixed with water to keep everything in place. Unlike the living cherry blossoms, or sakura, that come and go so quickly each year in Japan, these pretty minerals live on as long as the glue holds.
"Although the sakura are ephemeral in their beauty, lasting only a few weeks each year,” says Rakovan, "their image has been set in stone in the sakura ishi of Kameoka."