What do pearls and Tums antacids have in common? They are made from the same substance Aragonite is from - calcium carbonate. Aragonite is another element not stable at low pressures and temps but does form due to "magnesium poisoning". Solutions which are supersaturated w/ calcite can't precipitate if there is also a high concentration of magnesium thus allowing aragonite crystals to form. Argonite can form in delicate crystals or in horizontal layers (like this example) and has a hardness of 4.