You can only dematerialize only those certificates that are already registered in your name and are in the list of securities admitted for dematerialization at NSDL / CDSL. All the scrips included in S&P, CNX, NIFTY and BSE SENSEX have already joined NSDL / CDSL. This list has more than 4,500 companies and is steadily growing. You can get an updated list of these companies from your DP or from the websites of the depositories viz. NSDL and CDSL.
Basically, there are two key points to note here. Firstly, the shares should be in your name. That means if you have bought in the physical form many years ago and shares are not yet transferred into your name, then first get it transferred into your name. In the past, the SEBI was offer transfer cum demat facility which is no longer available. So will have to first send the physical certificates to the company/registrar and get the shares transferred into your name. Once that is done, then you can send for dematerialization. The second issue is whether the stock is available for demat. You can get the list from the website of NSDL or CDSL and accordingly, you can check if the shares are available to be dematerialized. Companies are normally required to approach NSDL or CDSL to get their shares dematerialized and only then they will be eligible for shares to be dematerialized.