Biomineralization is the process of formation of functional biominerals in living organisms (bones, teeth, shells). When undesirable biominerals are formed, we are talking about pathological biomineralization (caries, stones, ...). Kidney stones are an example of pathological biomineralization that causes a chronic disease - urolithiasis. Nowadays, kidney stones are an increasingly frequent topic of conversation due to lifestyle, poor diet and various external factors. In 80% of cases, kidney stones are calcium oxalate. In kidney stones, calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD) predominate most frequently, while calcium oxalate trihydrate (COT) is rarely found in the composition of stones, but it is believed to be the first hydrate phase formed and then transforms into a more stable form. The aim of this work was to synthesize the unstable hydrate phase in as pure form as possible and to characterize the crystals obtained and study their properties using optical microscopy, infrared spectroscopy, X-ray diffraction and thermogravimetric analyzes. The syntheses were carried out at three temperatures (25, 36.5 and 48 ℃) and three ionic strengths (0.01, 0.05 and 0.1 mol dm-3). It was found that COT is formed at 25 and 36.5 ℃, while increasing the temperature to 48 ℃ produces COM regardless of the ionic strength.