The mother-of-pearl, or  Nacre, from sea shells was used to for various decorative purposes - to make buttons, accordion keys, fashion jewelry... but even if this is still done,

I've also read that in some regions ground sea shells are added to chicken feed, as a calcium supplement, just like crushed limestone is used for the same purpose elsewhere.

Oyster shells are often used as a replacement for gravel in south Louisiana and other parts of the Gulf of Mexico. There are no local gravel quarries, and oyster shells work quite well as light-duty aggregate (cheap parking lots, fill to raise elevation, and so on). In Louisiana it is quite common to use them as a road bed in lieu of gravel.

 They are used to remove heavy metals from water.

Using Sea Shells to Remove Heavy Metals from Water

Also some places are looking at them for an additive to concrete. Seashells a potential ingredient in structural concrete

Seashells can be used for civil construction activities. Here's how the waste shells were used for various purposes in Bayelsa, Nigeria

A Waste Management Perspective

Oyster shells can be recycled by returning them to the ocean to enrich natural oyster habitats.

Also shells can be made into calcium supplement pills.

Recycling Mystery: Oyster Shells - Earth911.com

Very interesting

thank you so much, Mr.Mushtaq Ahmad 

In Ayurveda Sea Shells are subjected to calcination (Bhasmikarana) and by this it is converted into Nano particles called Bhasma. It is called Shankha in Ayurveda. It is rich source of Calcium.

Shankha Bhasma Uses: 

It is coolant, improves skin color and complexion.

It is useful in the treatment of gastritis, abdominal colic, malabsorption syndrome, dyspepsia, vomiting, nausea, acne, etc.

Growing children, adolescents and post-menopausal women need more calcium. It provides natural calcium and vitamin C for the development and maintenance of healthy bones, teeth and cell membrane.

It easily absorbs and compensates the calcium requirement of the body.

It relieves the body from the condition of calcium deficiency without any side effect like constipation and acid formation in stomach.

Chemical composition: Calcium carbonate – CaCO3

Shankh Bhasm dosage:

250 mg – 500 mg once or twice a day before or after food or as directed by Ayurvedic doctor. It is traditionally administered along with honey, lemon juice, Triphala kashaya etc.

Shanka Bhasma preparation, ingredients, how to make:

It is prepared from purified Conch shell. There are many different methods of preparation. Any one method can be used. Here is one such method.

Purified Shankha powder is ground with aloe vera juice or lemon juice, made into disc shaped cakes, dried, burnt in absence of air, in 800 – 900 degree celsius, allowed to self cooling. This process is repeated twice to obtain Bhasma.

Thanks Mr. Vaidya Vasant Patil