According to Pagat.com, dominoes were invented in China in the 1000s by Keung Tai Kung. As explorers, like Marco Polo, visited the East, they brought back many inventions and customs found there. Amongst them were various games ranging from dominoes to mahjong to skipping rope. The dominoes of that long bygone era were made out of ivory obtained from the tusks of elephants. As ivory grew in value, people started killing elephants just to gather the ivory. You can determine if dominoes are made out of ivory by performing a quick test on them.
Set up a very bright light near the area you will be checking the dominoes so you have as much visibility as possible. Direct sunlight is a good substitute for lighting.
Place one of the dominoes in your palm to feel the weight of the piece. If it feels light and insubstantial it is not ivory, as ivory is a dense and weighty material.
Feel the finish on the domino to determine the texture. Any ivory domino will feel smooth and creamy with very gradually rounded corners. Sharp, clearly angled corners are likely to indicate plastic or other synthetic materials.
Inspect the surface of the domino under a magnifying glass. Look on the sides of the piece for a fine grain. All forms of genuine ivory have a natural pattern reminiscent of the inside of a cut piece of wood.
If you find a grain pattern, you can bring it to a jeweller to help you identify which form of ivory you have.
Do not deal in the trade or purchase of any ivory that is prohibited by law in the country you live in.