Last week, I invited Fool co-founder David Gardner into our brand-new Motley Fool studio space here in Alexandria, Va. We spent nearly an hour talking about the state of the retail investor here in 2013, his investing philosophy, and his current thinking on a half-dozen specific stocks in his Supernova universe. (You can view the entire interview here.)

Among the companies we discussed were two of the 10 best-performing stocks of 2012, 3D Systems (NYSE:DDD) and Stratasys (NASDAQ:SSYS).

David recommended both of these companies last year, and still likes them (and in equal weights). David believes 3-D printing as an industry is going to be vital in the future -- not unlike the where the Internet stood 15 years ago. You can watch his full answer in the video below (run time: 3:56).