AbstractThe morphology of calcium oxalate monohydrate precipitates (COM, Ca(C2O4)·H2O, P21/c, whewellite) occurring as crystals or intergrowths, as well as distribution of crystal‐bearing idioblasts, have been studied for the first time in the bark of stone birch Betula ermanii from Sakhalin Island sampled in an area affected by mud volcanism and an unaffected typical forest environment taken for reference. The study addresses several issues (i) number and size of phytoliths and their distribution in different cell types; (ii) density of calcification in specific cells; (iii) habits of single crystals, twins, and complex intergrowths, as well as frequency of different morphologies and their relations. The trends of time‐dependent morphological changes in separately analyzed crystals and intergrowths record the evolution of COM morphology from nuclei to mature grains. Of special interest are the nucleation sites and features of organic and inorganic seeds and nuclei for COM phytoliths. The precipitation process and crystal habits are mainly controlled by supersaturation, and it is thus important to constrain the Ca distribution patterns in different bark tissues. The B. ermanii samples were analyzed by several methods: scanning electron microscopy (SEM) for the distribution patterns and micromorphology of COM precipitates and bulk Ca content in bark; electron probe microanalysis (EPMA) for the mineral chemistry of COM precipitates; inductively coupled plasma optical emission spectrometry (ICP‐OES) and inductively coupled plasma mass spectrometry (ICP‐MS) for trace elements in bulk bark and wood.Research Highlights The distribution and morphology of whewellite precipitates in the analyzed B. ermanii bark samples indicate that the aqueous solution was most strongly supersaturated with respect to the Ca(C2O4)·H2O solid phase at the parenchyma‐sclerenchyma boundary, where most of the COM spherulites are localized and often coexist with large single crystals and contact COM twins.