Yes, it is real. I researched a lot on this stone. This is a small black coloured and pale green coloured snake stone found near and around cobra snake. In South India, it is very common. It may be of different colour in different snakes, sometimes black and sometimes pale green colour. This stone glows in dark for few seconds. There is not much scientific study on this stone.
The snake catchers in south India remove this stone from snake nest. It is said that the shining of this stone is its natural property (like in Jugnoos), which might be evolved, so that this glow could attract other small insects, rodents and snake could eat them. In some countries this stone is used as anti-venom in two ways.
So, the second way above might be based upon the same principle. It is also said that if you have this stone, the snakes will stay away from you. The snakes get highly scared after watching this stone and try to move away from this stone (I couldn’t find the reason of this). There is not any published scientific research on this stone’s anti-venom property because doing this may shut the business of those companies who are making a hell lot of money from a single antivenom injection. The other reason of less research on this stone might be the danger (after all its about snake not about mouse) involved in this research. But in Hindu’s mythology named Shiv-Purana, it is said that the real NAG-MANI (snake pearl) is developed in snake after it completes hundred years without biting anyone and the same is so much powerful that it glows for lifetime, and its size becomes so large that it can be seen from far away with bare eyes, and the food (prey) itself start coming towards the snake by its attraction and the person who gets successful in possessing that NAG-MANI (snake pearl) from snake will become equally powerful and money itself start coming towards that person by different ways and all kind of snakes, evil stay away from him.
Cobra Pearl is actually like a Gem also known as Snake Stone and people out there used as folk medicine for snake bite.