It is a plastic resin that has been blended to a precise dosage with YAG phosphor. You can buy YAG phosphor easily from wholesalers, for example, phosphortech, a Georgia company, sells a variety of colors of phosphor. However, the trick is to precisely and uniformly blend it in with the plastic resin - the reason that it takes so much time and development money to make a light bulb like this.

Nice, who do you work for?