The first half of 2016 has been extremely kind to the Footsie’s fleet of diggers and drillers. And June’s Brexit vote pushed ‘safe-haven’ appetite for these companies still higher.

Fossil fuel giant Shell (LSE: RDSB) saw its share price explode 34% during January-July; diversified mining play Anglo American (LSE: AAL) enjoyed a 143% increase; while precious metals play Fresnillo (LSE: FRES) saw its value jump 132% during the period!

But I believe now could be the time for investors to lock-in some of these gains.

Crude concerns

Shell’s share value has been swept higher by a recovering Brent price. The Brent benchmark has remained steadfast around the $50 per barrel milestone in recent weeks, a staggering improvement from the multi-year lows of $27.17 punched in January.

Although hopes of an OPEC-led production freeze failed to materialise, subsequent output problems in Canada and Nigeria have allowed oil prices to maintain their strength.

However, signs that US producers are getting back to work — Baker Hughes rig data has shown the number of units in operation rising for four out of the past five weeks — allied with on-going reluctance elsewhere to reduce production could keep crude prices hemmed-in around current levels.

And this could prevent Shell from making further ground, particularly as the firm already deals on an elevated forward P/E rating of 27.4 times.

Steel struggling

Anglo American has been a major beneficiary of the surging iron ore price during the first half of 2016.

But rapidly-reversing commodity values suggests that Anglo American’s ascent may be built on shakier foundations. Like oil, much chatter has been doing the rounds that heavy speculative buying is the main driver of iron ore prices in recent months.

Indeed, rising international pressure on China to curb steel capacity casts a pall over iron ore demand for the coming years. The country plans to shutter 100m-150m tonnes of capacity during the next five years. And China’s stagnating construction sector also creates concern over imports of the steelmaking ingredient.

Given these worries, I reckon Anglo American’s huge P/E rating of 23.8 times leaves plenty of room for a serious stock price correction.

Precious play

It could be argued that Fresnillo’s earnings outlook isn’t fraught with such dangers.

The extreme uncertainty surrounding Britain’s withdrawal from the EU — and consequent ramifications for the entire global economy — could continue to fuel safe-haven demand for gold. Indeed, the metal hit $1,350 per ounce in Monday business, its highest level since March 2014. And further gains could pull Fresnillo still higher.

Still, Fresnillo isn’t immune to the perils of a colossal cooling in the global economy. Of course the Mexican digger is the world’s largest producer of silver, a market already whacked by falling demand from critical sectors like photography.

And an enormous prospective P/E rating of 68.6 times leaves Fresnillo looking extremely top heavy, in my opinion. I believe the commodities segment is still fraught with danger.

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Royston Wild has no position in any shares mentioned. The Motley Fool UK has recommended Royal Dutch Shell B. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.