Magnetite (Fe 3 O 4 ) crystals have been synthesized from the natural iron sands of Tegal Lenga Beach with HCl (37%) as a solvent. Tegal Lenga Beach is a stretch of beach located in Kalisada Village, Seririt District, Buleleng Regency, Bali Province, Indonesia. Iron sand samples were obtained by extracting local natural sand from Tegal Lenga Beach by magnetic separation method. The iron sand sample was ground with agate mortar for 15 hours, then washed and rinsed with deionized water (DI water). Furthermore, the fine powder sample of iron sand was dried on a hotplate at a temperature of 80˚C until all the water had evaporated. Eighty grams of fine iron sand powder dissolved in 200 mL HCl (37%) while stirring with a hot plate magnetic stirrer at a temperature of 100˚C with a rotating speed of 600 rpm for 30 minutes. This solution was filtered through Wattman filter paper No. 42; then the filtrate was dried using a hot plate at a temperature of 100˚C to form a crust. This crust is then ground in an agate mortar until it becomes a fine powder of iron sand. Furthermore, the iron sand fine powder filtrate sample was divided into four equal parts which were then calcined at temperatures of 300˚C, 400˚C, 450˚C, and 500˚C for 30 minutes, respectively. It was found that the fine powder of iron sand from the natural sand of the Tegal Lenga Beach consisted mostly of 84.26% metal oxide magnetite. The synthesized magnetite crystal has a tetrahedral structure with lattice parameters of a = b = (5.927 ± 0.0180) Å and c = (16.774 ± 0.0145) Å. Magnetite crystals have granular and lumpy grains with an average grain size of (94,560 ± 10,1397) μm. In relation to the values obtained from the hysteresis curve, namely the remanent magnetization of 9.6672 emu/gr, saturation magnetization of 45.3491 emu/gr, coercive magnetic field strength of 0.0242 T, and saturation magnetic field strength of 0, 2938 T, then this magnetite crystal is classified as a weak ferromagnetic material.