Limestone itself is not something you should eat straight from the ground, but its primary mineral, calcium carbonate, is widely used in food production and sold as a dietary supplement. The difference comes down to purity. Raw limestone contains variable amounts of clay, silica, heavy metals, and other contaminants that make it unsafe to consume. Food-grade calcium carbonate, refined from limestone or similar mineral sources, is a recognized food additive used in everything from fortified cereals to antacid tablets.
What Limestone Actually Is
Limestone is a sedimentary rock composed mostly of calcium carbonate. It forms over millions of years from the compressed remains of marine organisms like shells and coral. Because it’s a natural rock, its composition varies by location. A piece of limestone from one quarry might be nearly pure calcium carbonate, while another could contain significant amounts of lead, arsenic, or other heavy metals picked up from surrounding geology. This unpredictability is why eating raw limestone is a bad idea, even though the core mineral is harmless in purified form.
How Calcium Carbonate Shows Up in Food
Once limestone is processed and purified to food-grade standards, calcium carbonate becomes one of the most common food additives in the world. The European Food Safety Authority has evaluated its safety under the additive code E170, approving it for use across food categories including infant formula. In the United States, the FDA permits calcium carbonate as a coloring agent in dietary supplement tablets, soft and hard candies, mints, and chewing gum inks.
Beyond coloring, calcium carbonate serves several practical functions in food manufacturing. It acts as a firming agent, an anti-caking powder, and a calcium fortifier. If you’ve ever taken a calcium supplement or chewed a Tums tablet, you’ve eaten refined limestone. Calcium carbonate supplements contain about 40% elemental calcium by weight, making them the most concentrated and cost-effective form of supplemental calcium available. The tradeoff is that calcium carbonate needs stomach acid to be absorbed properly, so it works best when taken with a meal.
Limestone’s Role in Making Tortillas
One of the most important culinary uses of limestone-derived products goes back thousands of years. Nixtamalization is a pre-Columbian process in which dried corn kernels are cooked in a solution of calcium hydroxide, a chemical made by heating limestone and mixing it with water. This alkaline bath transforms corn in ways that matter nutritionally and culinarily. It loosens the tough outer hull of the kernel, makes the protein in corn more digestible, and releases niacin (vitamin B3) that would otherwise pass through your body unused.
Without this limestone-derived treatment, populations that depend heavily on corn risk pellagra, a serious niacin deficiency disease. The process also gives the dough its distinctive flavor and pliable texture. Tortillas, tamales, pozole, tostadas, nachos, and corn chips all depend on nixtamalized corn. If you’ve eaten any of these foods, you’ve consumed a product made possible by limestone chemistry.
Why Some People Eat Raw Earth
The practice of deliberately eating soil, chalk, clay, or limestone is called geophagy, and it occurs across cultures worldwide. In parts of South Africa, for example, geophagy is prevalent across multiple provinces, with people consuming earthy materials for reasons ranging from cultural tradition to pregnancy cravings to perceived medicinal benefits. Some researchers believe the behavior may reflect an instinctive attempt to supplement minerals like calcium or iron, or to soothe gastrointestinal discomfort, since clay and calcium carbonate can neutralize stomach acid much like an antacid.
When geophagy involves unprocessed natural materials, though, it carries real risks. Unrefined earth can contain parasites, bacteria, and toxic metals. Eating large amounts can also block absorption of iron and other nutrients, leading to the very deficiencies the behavior may be trying to correct. This is distinct from the clinical condition called pica, where people compulsively eat non-food substances. Pica sometimes involves chalk or limestone and is associated with iron deficiency, pregnancy, or certain developmental conditions.
Calcium Carbonate as a Supplement
For people considering calcium carbonate specifically as a source of dietary calcium, a few practical details matter. Because it contains 40% elemental calcium by weight, you need fewer tablets compared to calcium citrate, which is only 21% calcium. Calcium citrate absorbs more easily and doesn’t require food for absorption, making it a better choice for people with low stomach acid or those who prefer taking supplements on an empty stomach. For most people with normal digestion who take their supplement with meals, calcium carbonate works well and costs less.
Your body can only absorb about 500 milligrams of calcium at a time, so splitting doses across the day improves how much you actually retain regardless of which form you choose.
The Short Answer
Purified calcium carbonate derived from limestone is safe, effective, and already in many foods you eat. Raw limestone from the ground is not food-grade and carries contamination risks that make it unsuitable for eating. The mineral itself is fine for your body. The delivery method is what matters.