A glance at the biggest holdings of Warren Buffett's Berkshire Hathaway (NYSE:BRK-A)(NYSE:BRK-B) reveals a wide range of familiar names like Coca-Cola, Wells Fargo, and IBM to name a few. But a few stick out because they're so unknown, and one likely raises more eyebrows than any.

It causes investors to ask the question, why is Chicago Bridge & Iron Company N.V.(NYSE:CBI) one of the biggest Warren Buffett stocks?

The history
To start, it's vitally important to clarify we do not know if Buffett himself was the one who made the purchase decision to begin buying CB&I in the beginning of 2013. In his 2011 letter to Berkshire shareholders Buffett importantly noted:

When our quarterly filings report relatively small holdings, these are not likely to be buys I made (though the media often overlook that point) but rather holdings denoting purchases by Todd or Ted.

Todd Combs and Ted Weschler are two of Buffett's understudies who each manage an equity portfolio for Berkshire Hathaway that was worth more than $7 billion at the beginning of this year. And just because they're not Buffett doesn't mean the companies they invest in should be discounted, as Buffett added in this year's letter to shareholders:

In a year in which most equity managers found it impossible to outperform the S&P 500, both Todd Combs and Ted Weschler handily did so. ... I must again confess that their investments outperformed mine. (Charlie says I should add "by a lot.") If such humiliating comparisons continue, I'll have no choice but to cease talking about them.

Encouragingly he concluded the discussion on them by saying, "Both men have Berkshire blood in their veins."

So, even if we don't know if it's one of Buffett's stocks, that CB&I is in the massive Berkshire equity portfolio means it is one we should pay attention to.

But how did it get there? It all started during the first quarter of 2013, when it was revealed Berkshire had amassed a $400 million position in CB&I, accumulating 6.5 million shares.

As you can see, the buying continued in the second quarter of that year, as Berkshire Hathaway added almost 50% more to its stake. The position held steady for a year, but in the second quarter of 2014, as the stock price of CB&I fell more than 20%, another 1.1 million shares were purchased, representing a growth of 12%.

Although we don't yet know where the holdings of CB&I by Berkshire stand after the conclusion of the third quarter, it will be very curious to monitor, as its stock price continued to dive, falling another 15%.

The reason for the investment
Finding a definitive rationale to understand why Berkshire now owns more than 10% of Chicago Bridge & Iron is easier said than done.

However, taking a step back and considering what CB&I does provides a bit of an insight. It is an energy infrastructure company that provides a wide range of services and solutions across the entire lifecycle of a project.

Remember it was just a few months ago when Buffett told Bloomberg that Berkshire had "poured billions and billions and billions of dollars in retained earnings, and several billion of additional equity [into the energy businesses]" and it planned "to keep doing that as far as the eye can see."

So an investment in CB&I from simply an industry perspective is sensible to Berkshire. But just liking the industry doesn't warrant an investment in a company in that industry.

As fellow Fool Sean Williams notes, there are a number of understandable reasons why anyone looking toward a value investment -- which Buffett has always championed -- should consider an investment in CB&I.

This year it has delivered strong results, and its adjusted net income per share stands at $3.74 through the first nine months of 2014. This represents a gain of 26% relative to the same period in 2013. In addition its revenue is up 13%.  

But the market isn't too pleased with this impressive growth, as it trades at a price-to-earnings ratio right at 10 -- well below where it has typically been valued over the last five years:

CBI PE Ratio (TTM) Chart

With a strong business, impressive results, and a reasonable valuation, it's no wonder why Chicago Bridge & Iron is one of the biggest Berkshire Hathaway stocks.