I have squeezed myself into a tiny, abandoned council flat in the backlands behind Elephant and Castle, in south London. Abandoned, but mysteriously colonised. Who knows who once inhabited this 1960s bedsit? But an alien substance has taken it over. The entire flat - walls, ceiling, fixtures, fittings - has sprouted not mould, not fungus, not graffiti, but something far stranger and more beautiful. Hard, glittering, jagged, bright-blue crystals.

A joint product of the fecund Artangel and Jerwood organisations, this place of chilly, sinister beauty is Roger Hiorns’s Seizure installation. Every day, shoals of people arrive, get kitted out with gumboots (it’s wet in there) and gloves, and queue patiently to be allowed into this modern-day grotto, two or three at a time. It has now had its run extended to the end of the month. I’m prepared to bet that the queue on the last day will stretch round the block.

What to make of it? To judge by the exclamations of my fellow crystal pilgrims, the principal response is simple wonder at the way something so perfect, so intense in its depth of colour, can transform a prosaic interior. It’s dark in there: just a couple of dim light bulbs to see by. The crystal growths are surprisingly big, pyramidal, sharply pointed, like a dream of Manhattan skyscrapers seen from above. They jostle each other, grow into and over each other. They could fill this space so it became a solid block of mineral. You imagine what it might be like to be trapped in there, the doorway grown over. You might somehow be slowly pulped by the advancing crystalline needles, until you were no more than an impurity at the heart of some massive rock formation. In the meantime, this small concrete, brick and plaster-board dwelling has become a bejewelled cave. And it is not actively frightening. The crystals cannot grow in air. You have time to get out.

Very little was left in the flat before Hiorns’s transformation, but what there was - a hanging lamp, a bath - has been taken over completely by the crystal growth. And so, rather as with Doris Salcedo’s Shibboleth, the famous crack in the floor of Tate Modern last year, you pose the obvious question: how on earth did he do it?

The answer is simple and at the same time remarkably difficult to achieve on this scale. Anyone who has had a chemistry lesson knows how you grow copper-sulphate crystals: make a supersaturated solution by dissolving lots of them in hot water, then, as it cools, they recrystallise, growing on whatever you dangle in the solution. They are a lovely shape and colour, and they can’t help it. The geometric forms they grow into, and the colour they adopt, is locked into their chemical composition. Crystalline formation just happens, given the right chemical conditions. So Hiorns decided his test tube would be a building. He would instigate the process, then let the crystals take over.

Hiorns has a bit of a thing about the brutalist architecture of this period. He likes it, I think. He also likes transforming objects, not just with crystals, but with foam, perfume, even fire. He has crystallised models of cathedrals, BMW car engines. They become sculpture with a wave of the artist-chemist’s wand. A habitable interior, however, is more of a challenge. It might not have worked at all. But Hiorns has read his JG Ballard. He knows The Crystal World.

This is a rather nice little two-storey courtyard of bedsits, but the council wants to demolish it, so Hiorns had a free hand. He sealed a ground-floor flat - turning it into a huge tank - and filled it with 75,000 litres of hot, supersaturated copper-sulphate solution, poured in through holes in the floor of the flat above. Then he waited for it to cool, pumped out the remaining liquid and broke back into the sealed flat to see what had happened. It had worked. The crystals had turned the bedsit into a kind of paradise.

Your prime sensation as a visitor is that of the explorer. It’s like venturing into Tutankhamun’s tomb, or being the first into those time-capsule homes or forgotten sweet shops in boarded-up buildings. In all of those, though, there is evidence of human life, former activity. Here, there is something else. As Hiorns points out, even the artist disappears. The substance takes over and follows its own logic. It makes itself.

That is the power of Seizure. We are in the presence of something both beautiful and incredibly powerful, a chemical and physical force. It doesn’t need us. It doesn’t know it’s there, or that we are in it, and what drives its formation is a power that will outlast us all. Hiorns reminds us of our own immense fragility.

Seizure, 157 Harper Road, SE1, until Nov 30, Thu-Sun

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010