Source: Flickr via user Steven Depolo.

The stock market is perhaps the most powerful wealth-building instrument in existence today. History has shown that investors who have simply plowed money into even broad-based index funds over long periods of time have enjoyed big returns. 

Having said that, a select few stocks have transformed their early shareholders into millionaires seemingly overnight. While it's always a good idea to take the long view toward investing, our Motley Fool contributors think Zoe's Kitchen (NYSE:ZOES), MercadoLibre (NASDAQ:MELI), and Celldex Therapeutics (NASDAQ:CLDX) may have the right ingredients to generate life-changing gains in relatively short periods of time. Here's why. 

Tim GreenIf anything can be learned from Chipotle, it's that a fast-casual restaurant chain has the potential to provide tremendous returns for investors if it can manage to grow its store count into the thousands. Many chains try and fail, hitting a wall after a few hundred stores are opened. But occasionally, a company breaks through.

Zoe's Kitchen, a small chain that serves Mediterranean-style dishes, is still a long way from that level of success, but I think the company has a good chance of growing into a national chain. As of September, Zoe's operated 158 restaurants, mostly in southern states, with the focus being on fresh and healthy ingredients. Each restaurant generates about $1.5 million in annual sales, significantly higher than the $1.06 million the average Chipotle location generated when that chain had a similar store count to Zoe's.

Source: Zoe's.

In the long run, Zoe's hopes to open a total of 1,600 domestic locations, and if the company can also continue to grow its annual per-store sales to $2.5 million, where Chipotle sat in 2014, total revenue would increase by more than a factor of 20. While shares of Zoe's currently trade at a premium, about three times current sales, the price won't matter if the company hits its long-term target. Zoe's could fail, of course, and become yet another fast-casual chain that falls short of its own expectations. But no risk, no reward.  

Andres Cardenal: MercadoLibre is the undisputed e-commerce leader in Latin America, an industry offering enormous potential for growth over the years ahead. In addition to matching buying and sellers of all kinds of products, the company owns its own digital payments platform, MercadoPago. MercadoLibre is also expanding toward areas such as advertising and logistics to provide a better experience to both buyers and sellers in its marketplace.

Source: MercadoLibre.

Economic headwinds are considerably dragging on MercadoLibre's financial performance, but the company is still delivering rock-solid growth when leaving external considerations aside. Total sales during the third quarter grew 15% in U.S. dollars and buy a much stronger 68% in local currencies. The amount of units sold in MercadoLibre's platforms jumped 26% year over year. 

MercadoLibre has been hurt by economic instability and currency devaluations in many of its biggest markets over the last several years. However, the political tide seems to be changing in Latin America; key countries in the region could embrace more reasonable economic policies in the future, and MercadoLibre stands to benefit substantially from this change.

Argentina recently elected a new market-oriented president, and populist leaders in Brazil and Venezuela are facing increasing discontent from voters due to rampant corruption and failed economic policies. While it's far too early to say that Latin America is definitively on the path to sustainable economic development, the political environment seems to be clearly improving.

George Budwell: Small-cap biotech stocks have proven to be fertile ground when it comes to producing jaw-dropping gains. In the same breath, they are also known to crater on a moment's notice, only to never be heard from again. So, it's important to choose your speculative biotech stocks wisely.

Among this group, I think the clinical-stage biotech Celldex Therapeutics is a name that particularly stands out from the crowd because the company is building out an impressive platform in the high-growth cancer-drug space. For example, Celldex recently announced that its experimental-stage vaccine, Rintega, produced a marked overall survival benefit in patients with EGFRvIII-positive recurrent glioblastoma, a particularly aggressive form of brain cancer.

Source: NIH.

With a second interim analysis for Rintega's late-stage study as a treatment for newly diagnosed EGFRvIII-positive glioblastoma on track for early 2016, Celldex could potentially have its first product on the market within the next 12 months -- that is, if this late-stage study is halted early for efficacy reasons.

But what's really exciting about this stock is that Celldex sports a well-diversified portfolio of high-value clinical-stage oncology products. Aside from Rintega, Celldex has the antibody-drug conjugate Glembatumumab vedotin, targeting tumors that overexpress gpNMB, and a number of other earlier stage product candidates with blockbuster potential.

While it's probably fair to say that most of these experimental products will ultimately fail to reach the market, Celldex does offer investors multiple shots on goal at breaking into the $50-billion-a-year-plus cancer drug industry. That's why I think this stock has tremendous potential for investors who are willing to tolerate short-term volatility.