Calcium Carbonate
<p><b >Calcium carbonate</b> is a <a href="http://en.wikipedia.org/wiki/Chemical_compound" title="Chemical compound" >chemical compound</a> with the <a href="http://en.wikipedia.org/wiki/Chemical_formula" title="Chemical formula" >formula</a> <a href="http://en.wikipedia.org/wiki/Calcium" title="Calcium" >Ca</a><a href="http://en.wikipedia.org/wiki/Carbon" title="Carbon" >C</a><a href="http://en.wikipedia.org/wiki/Oxygen" title="Oxygen" >O</a><sub >3</sub>. </p>
<p>
It is a common substance found in <a href="http://en.wikipedia.org/wiki/Rock_(geology)" title="Rock (geology)" >rocks</a> in all parts of the world, and is the main component of <a href="http://en.wikipedia.org/wiki/Seashells" title="Seashells" class="mw-redirect" >shells of marine organisms</a>,<a href="http://en.wikipedia.org/wiki/Snail" title="Snail" >snails</a>, <a href="http://en.wikipedia.org/wiki/Coal_ball" title="Coal ball" >coal balls</a>, <a href="http://en.wikipedia.org/wiki/Pearls" title="Pearls" class="mw-redirect" >pearls</a>, and <a href="http://en.wikipedia.org/wiki/Eggshell" title="Eggshell" >eggshells</a>. </p>
<p>
Calcium carbonate is the active ingredient in <a href="http://en.wikipedia.org/wiki/Agricultural_lime" title="Agricultural lime" >agricultural lime</a>, and is usually the principal cause of <a href="http://en.wikipedia.org/wiki/Hard_water" title="Hard water" >hard water</a>. It is commonly used medicinally as a <a href="http://en.wikipedia.org/wiki/Calcium" title="Calcium" >calcium</a> supplement or as an <a href="http://en.wikipedia.org/wiki/Antacid" title="Antacid" >antacid</a>, but excessive consumption can be hazardous.</p>