Mid-cap stocks can be a great way to find wonderful growth potential, while also avoiding some of the risks with small-cap growth stocks. After all, the best mid-cap growth stocks are former small caps that have established themselves and built billion-dollar businesses with plenty of room left to grow. 

With that in mind, three of our top contributors think you should take a closer look at Tractor Supply Company (NASDAQ:TSCO)Royal Gold, Inc. (USA) (NASDAQ:RGLD), and First Solar, Inc. (NASDAQ:FSLR). Not only have all three of these companies built strong, established businesses already, but they still have great growth prospects that could see them reach large-cap status in short order, lifting investors' portfolios along the way. 

Man drawing a chart showing growth.

Image source: Getty Images.

A beaten-down retailer

Demitri Kalogeropoulos (Tractor Supply): Rural lifestyle retailer Tractor Supply has seen its market capitalization sliced almost in half since early 2016 to $7.2 billion. There's room for this industry niche leader to climb back above $10 billion, though.

Sure, customer traffic fell last quarter for the first time in a decade, raising fears that Tractor Supply is getting caught up in a long-term retailing slump. But weather and calendar shifts played big roles in that slowdown, and so a more seasonable spring likely brought improved operating trends with it. Management frequently points out that the company's sensitivity to weather makes six-month periods, rather than quarterly periods, more useful in evaluating the business.

Looking further out, Tractor Supply is making major improvements to its digital offerings that should keep it ahead of the trend toward e-commerce shopping. It's rolling out a national "buy online, pick up in store" program, and the early results point to not only increased digital purchasing but also extra spending at stores. Its sometime-rival Home Depot has seen similarly strong gains by adding the same functionality. And like the home-improvement giant did a few years ago, Tractor Supply is now adding a distribution center to its network that will put nearly all of the U.S. population within a two-day online delivery window.

But Tractor Supply's biggest advantage is its unique ability to meet the needs of a particular subset of customers: land, pet, and livestock owners located outside of metropolitan areas. Its recent sales and profit drops are worth watching, but they don't indicate that the retailer has lost touch with this shopper base. Instead, I'd expect Tractor Supply to make the right adjustments that put it back at its almost double-digit annual sales growth pace and help the stock recover from the past year's slump. 

A golden opportunity 

Sean Williams (Royal Gold): When you look for mid-cap stocks that could balloon into a large-cap valuation, you're liable to see a big focus on growth stocks. As for me, I prefer the slow and steady growth provided by precious-metals royalty and streaming company Royal Gold.

Royal Gold has a pretty huge advantage as a royalty and streaming company: It doesn't deal with day-to-day mining costs. Instead, the company provides funding to mining companies looking to expand an existing mine or develop a new mine in exchange for a percentage of production at an exceptionally low cost over a long period of time. Through the first nine months of its fiscal year, general and administrative expenses and cost of sales (the aforementioned low payout to miners for their production) have totaled just $23.4 million and $67.6 million, respectively, compared to $331.9 million in revenue.  

It is worth noting that Royal Gold's business model does place it at the mercy of its producers. Though there's a major plus of not having to deal with day-to-day mining costs, there's no control over day-to-day mining operations with its partners, and hiccups (be they related to weather, equipment, or labor) do occur from time to time. Thankfully, it has enough deals in place that weaker production at any one of its producers won't wreck its quarterly results.

The long-term outlook for precious metals also looks promising. Even with rising interest rates, which makes interest-bearing assets more attractive and tends to hurt gold and silver prices, a plethora of other factors are buoying precious metals and could push them higher. Higher inflation levels, uncertainty surrounding Brexit and the Trump presidency, constrained precious-metal supply, and healthy demand are all reasons that I think gold has a decent chance of heading to $2,000 an ounce.

Ultimately, what you get with Royal Gold is a company with very high margins ($75.6 million in net income through three quarters on $331.9 million in precious-metal sales) that's currently paying a 1.2% dividend yield. If it keeps making smart long-term deals, this company could double in value to $10 billion, placing it in large-cap territory.

A solar leader

Jason Hall (First Solar, Inc.): Like Tractor Supply, solar panel maker First Solar has seen its stock price and market capitalization fall sharply over the past year or so. But unlike Tractor Supply, whose stock price has suffered over fears that its stores would begin facing the same struggles as other traditional retailers, First Solar is dealing with a temporary cyclical slowdown in demand for its solar panels, not a permanent shift such as what retailers are facing. 

And to some extent, the market has already started to realize it was undervaluing First Solar. 

FSLR Chart

FSLR data by YCharts.

Yet even after an incredible 45% gain in barely more than seven weeks, there's still a lot to like about First Solar as an investment for the long term. 

To start, its balance sheet is incredibly strong, with its net cash and short-term investments of more than $2 billion, worth more than 40% of the company's current market value. 

This incredible capital position means First Solar can continue to invest in its already industry-leading panel technology while many competitors are cutting costs, and that should help the company build on its competitive advantages. For patient investors willing to ride out the weak side of the solar cycle, it should pay off with wonderful returns. 

After all, solar remains one of the biggest long-term growth opportunities investors can find. There are decades of potential growth -- and on a global scale -- for First Solar.