Fingernails are made of a tough protein called keratin (from the Greek word ‘Kera’, meaning horn). Keratin is also what animals’ hooves and horns are made from. Most animals have a supportive bone structure in their horns, although rhinoceros horns are made completely of keratin compacted together. The only other biological material which has a similar toughness to keratinised tissue is chitin, the main component of exoskeletons belonging to arthropods.
The half-moon shape that you can see at the bottom of your nail (apart from maybe your little finger) is called the lanula. This is a group of cells that produce keratin and other living cells. As these living cells are pushed forward by newer cells, they die and merge with the keratin to become keritinsed. They then become flattened, stiff and known as your fingernails.
Sam Furniss, Science Museum