What happened
The FTSE 100 slipped 0.31 percent to 10,799.16 points on Thursday 13 August, underperforming European markets that mostly traded higher. The index had opened firmer after stronger than expected UK GDP figures, but the early gains evaporated as heavy selling hit the mining companies that dominate the London market.
The basic resources sector, meaning the miners that dig up copper, iron ore, gold and other raw materials, is the single biggest weight in the FTSE 100 and lost roughly 2.9 percent as metal prices weakened and shares retreated from recent record highs.
The casualty list was long. Antofagasta, the Chile focused copper producer, was the worst blue chip performer, down 4.79 percent. Rio Tinto fell 4.08 percent, silver miner Fresnillo dropped 3.81 percent and Anglo American slipped 2.59 percent.
A firmer pound added a second drag. Sterling strengthened after the GDP release, and because FTSE 100 companies earn most of their revenue overseas, a stronger pound shrinks the sterling value of those foreign earnings.
Why it matters
London is unusually exposed to this kind of move. Miners and other commodity producers make up a far bigger share of the FTSE 100 than of the S&P 500 or major European indices, so a bad day for metals is automatically a bad day for the UK benchmark, whatever the domestic economy is doing.
The pullback also says something about the commodity rally itself. Metal and mining shares have hit record highs this year on war disrupted supply and strong demand, and days like this show how much profit is sitting in those positions waiting to be taken.
The sterling effect highlights a quirk every UK investor should understand: good UK economic news can push the FTSE 100 down. Strong GDP lifts the pound, and a strong pound mechanically reduces the value of the roughly three quarters of FTSE 100 revenue earned abroad.
Explained simply
The FTSE 100 is like a cargo ship where the heaviest containers are stacked with copper and iron: when that cargo shifts, the whole vessel lists, even if every other container is perfectly secure.
An index is just a weighted average of its member companies, and weighting is the key word. Giants like Rio Tinto and Anglo American count for far more than a mid sized retailer, so a 3 to 5 percent fall in a handful of miners can sink the whole index even when most of the other 90 plus companies are flat or up.
Metal prices drive mining shares almost mechanically. A miner has largely fixed costs for digging, so when copper falls a few percent, profit falls much faster, and the shares amplify the move.
Add the currency effect, where a rising pound shrinks overseas profits on translation, and you get days like Thursday: a healthy economy, a rising pound, and a falling stock index all at once.
What it means for you
If you hold a FTSE 100 tracker in an ISA or workplace pension, Thursday cost you about a third of a percent, a routine wobble rather than a warning. Broad UK funds hold all sectors, so banking, pharma and consumer holdings cushioned the mining hit.
Anyone holding individual mining shares felt it far more sharply, with Antofagasta down nearly 5 percent in a day. That gap between a 0.3 percent index dip and a 5 percent single stock drop is the clearest advert for diversification you will see this week.
For pension savers drip feeding monthly contributions, a dip in a fundamentally strong market simply means this month you buy the same assets slightly cheaper.
If your fund is FTSE 250 based instead, it likely held up better: the more domestic mid cap index carries far less mining weight and benefits more directly from the stronger UK growth data.
The bigger picture
Mining shares remain among the best FTSE performers of 2026, lifted by supply disruption from the Gulf conflict and resilient global demand, so one down day barely dents the trend. The question is whether metal prices are peaking or merely pausing.
Watch Chinese demand data and any news from the Strait of Hormuz. Those two forces, more than anything happening in Britain, will decide where the commodity heavy FTSE 100 heads next.
