This powerful combination fueled investor enthusiasm that the Third Industrial Revolution -- a term coined by The Economist in April 2012 -- was just around the corner.

The following chart illustrates how the largest 3D printing stocks by revenue and market capitalization, 3D Systems (NYSE: DDD  ) and Stratasys (NASDAQ: SSYS  ) , have performed since The Economist article was published. Suffice it to say, it's been a wild ride.

Looking back, it appears that investor optimism toward 3D printing stocks got pushed to extremes, and created an environment in which they couldn't live up to their ever-growing expectations. However, despite the volatility that has ensued during the last three years, the long-term prospects of the 3D printing industry remain intact, which is a good reason for investors to keep an eye on 3D printing stocks.

According to Wohlers Report 2015, a leading 3D printing industry insights publication, the 3D printing industry generated $4.1 billion in worldwide revenue in 2014, and is expected to grow by more than 31% per year until 2020, eventually surpassing $21 billion in annual revenue.