By Sue Nelson
BBC Radio 4
Quartz, like sand, is made up of oxygen and the stuff of chips: silicon
Spruce Pine, a modest, charmingly low-key town in the Blue Ridge mountains of North Carolina, is at the heart of a global billion-dollar industry.
Although this Mitchell County community calls itself the Mineral City, with just 2,000 residents one could dispute the city status. But when it comes to minerals, Spruce Pine has definitely undersold itself.
The jewellery shops, highlighting local emeralds, sapphires and amethysts, hint at the riches. The mountains, however, contain something far more precious than gemstones: they are a source of high-purity quartz.
This ultra-pure mineral is essential for building most of the world's silicon chips - without which you wouldn't be reading this article.
Geologist Alex Glover, of Active Minerals International, drove me to a disused mine to see this quartz for myself. Our jeep bumped across dried creek beds for miles until we reached two cathedral-like caverns of rock at Hoot Owl mine.
The rocks contain feldspar, silvery flakes of mica, flashes of garnet and smoky veins of quartz. "Fifty years ago men were throwing away the quartz," explained Mr Glover.
"But now it's prized and quartz is the high value item. These are the only places that this quartz is found on the planet."
Spruce Pine quartz is considered the best in the world and can sell for up to $50,000 (£30,000) a tonne.
It is made, like all quartz, of silicon and oxygen but the process of making a computer chip does not rely on its silicon; that can be obtained from common sand.
The clue to why quartz is needed is in the process of making a silicon wafer. These wafers are CD-sized slices of silicon upon which the chips are then etched with electronic circuitry.
To make wafers, a seed crystal of silicon is heated to high temperatures in a giant mixing bowl until the molten, silvery metal can be stretched slowly upwards.
"It looks like a long cylinder, a bit like a salami," said Bob Carland, director of the Minerals Research Laboratory at North Carolina State University.
"As it comes out of this bowl, it all has one crystal and so everything is aligned the same way. The metal cylinder is then laid down and cut with a diamond saw into slices of salami - in this case it's slices of silicon wafers."
For these wafers to be made into silicon chips, the mixing bowls or crucibles must be as "clean" as possible.
Silicon wafer manufacture starts with a "seed" crystal of the element
"Any slight defect on the inside of that crucible will be transmitted and get sliced up into the chips," Mr Carland said.
"The amount of impurities in that chip is incredibly important. People producing these wafers will then have a lot of rejects so it's important that the chemistry of that bowl is near perfect."
Spruce Pine's high-purity quartz fits this requirement. It not only gets used for crucibles, but for benches and other instruments that produce the chips.
These chips are essential for today's modern world, but outside the industry, few people are aware of its dependence on quartz.
Unimin, one of the main mining corporations in the area, prefers to remain modest. It politely declines any interview, unwilling to reveal how it extracts quartz from the mountains.
Seen from the air, the scale of the operation reveals itself. The quartz mines are enormous, stretching down mountain faces in tiers of rock ending in pools of white sand.
It's a far cry from how the mountain folk made their living.
Sixty-eight-year-old Ira Thomas is a ninth-generation miner who used to dig up aquamarines and prospect for mica as a child.
He now runs the Spruce Pine Gem Mine, a jewellery shop near the scenic Blue Ridge Parkway, and is saddened by the loss of community among those who mine for quartz in industrial quantities.
"We have these huge corporations that have come in and bought out the little mom-and-pop operations," he said.
"These guys were just people like me that were lucky enough to lease a property that had minable quantities of whatever was on it."
Native Americans first mined these mountains but it was the arrival of a railroad in 1912 that gave later settlers the chance to exploit mica, feldspar and quartz on a much larger scale.
Shiny mica is chipped from rock made of feldspar and grey quartz
Today, in Mitchell County, one in 15 people works within the mining industry.
"That quartz plant just two miles down the road," he added, "is guarded like Fort Knox. That's a top-secret process. So we've got a serious commodity here."
He's right. The quartz plants are protected by security guards, gates and cameras and no one from the mining companies is allowed to talk to outsiders.
Lowell Presnell, historian and author of Mines, Miners and Minerals of Western North Carolina, isn't surprised.
"With the competition in the world today, they have to be really strict," Mr Presnell said, "because if they let their secrets out, somebody else is going to be doing this and they'll undercut their price."
There are also valuable jobs at stake. Since 2000, Mitchell County has lost a third of its manufacturing base and unemployment is at 14%.
North Carolina may be famous for its mountain folk and bluegrass music, but the mines are the only industry it has left.
So far, it is safe. Synthetic quartz is economically too expensive to take the place of Spruce Pine's high-purity mineral.
"It's the most valuable strategic square acreage on the planet," Mr Thomas said.
"Because the world runs on computers, we all know that now. And if we locked the gates to Mitchell County they could not make any more computers."
Sue Nelson presents Chips with Everything on BBC Radio 4 at 9pm on Monday 3 August.