Billy got in touch. We must meet and soon. I have a secret to reveal that only you can understand – a true exquis. As if we hadn't lost touch years ago.

The charge of his excitement ran through me. An exquis – our private language for something beautiful and rare not everyone would appreciate: a secret mulberry bush beyond the woods, heavy with sharply-perfumed fruit; a four-hundred-year-old diary in the chained library; a creased map of plans for a Blazing New World.

How could Billy have slipped from my mind? How could I have stopped loving someone as luminous and gentle and playful as him? What, I wondered, as I texted back Where and when? had become of me that I could have lost touch with the best of life?

We met in a tiny passageway café off Charing Cross Road, wedged in between a curio shop and an antiquarian bookseller's. Billy was already tucked into the corner, hair tied back, a tall tea-glass in a filigree holder on the table before him. He swirled its green leaves with a swizzle stick, like a magician concocting. As I approached, I smelled mint and honey steam rising from his glass, and then as we embraced that scent that Billy had, of air and earth, never stale but never soapy.

"Too long." I said, meaning since we last met but he looked confused. Time was nothing to him. So I said, "It's great to see you," and he smiled. I ordered mint tea. It arrived with pine nuts floating on the surface.

"Exquis?" I asked.

"Oh!" He breathed out long and low. I instinctively leaned in as he placed his hands delicately on the table, like a pianist about to perform. "Deep beyond the earth's crust," he told me, "miles below the reach of the deepest drill or mine shaft, there is a land entirely made of stone."

I waited. With Billy, anything was possible. During the decades we'd lost touch, he'd cropped up once, plastered all over the news for a week, having metal-detected an immaculate Anglo-Saxon hoard. A true exquis. But my second daughter had just been born and had breathing difficulties. We were in and out of hospital, no sleep. I barely followed his story. Never knew if it had made his fortune or belonged to the landowner or the nation. Hard to tell. He showed no signs of wealth or poverty. His shirt was plain white cotton, churned to a yellowy cream with age and washing. His jacket was thrift shop plaid.

"This land beneath our crust," he said, "this land is richly, densely populated, just like ours – I mean almost exactly like ours before the onset of more than the simplest mechanical engineering."

"O-kay..." I sipped my tea. Removed a pine nut from my teeth.

He nodded urgently. "And it is: complete! There are people ­­–– stone people ­­–– who dwell there in perfect unity with the earth. They have cities and markets and concert halls."

"Billy," I said gently.

"They move with tender, persistent grace."

"How –– did you –– come by this?'"

"Ah!" He lifted a finger, sudden brisk academic, defining a point of confusion. "To us, to our naked eye, hmm. There is a dilemma. Their air, such as they breathe, is also of stone."

His eyes met mine, gleefully keen and he lifted his trembling hands to force his point. "Every element within their land is unified: their air, their flesh, their habitat is all as one. Is stone. They are of stone and they move through stone, as one, entirely. In perfect harmony with this beautiful, beautiful stone."

"Oh."

"And," he dropped to the softest whisper here, "I have in my possession, a living, thriving, stone maiden from this land. I have her with me here. Now."

"You do?"

He leaned down and rustled in a bag-for-life at his feet. I had forgotten how plastic bags were always his ballast, tethering him to this world. He lifted a bundle from it, just larger than a human head, swathed in red and purple cloths which he began to unravel carefully. Despite his elegant fingers, Billy was not dexterous. He struggled. Threads from the torn strips had caught and he wouldn't use force to snap them but methodically tried unpicking each tangle. I glanced at my phone. 6:18. No way I'd make the 6:40. Shame on me for the thought. I'd cleared the whole evening to spend with Billy.

"Can I help?"

He smiled lovingly at me. "It's fine."

I sipped my tea. He unwrapped a bandage and began folding it neatly.

"At least let me do that." My voice was sharper than intended. He dipped his head, shot a wary glance up at me through stray strands of hair. Like a child. His fingers fussed and stumbled in an attempt to speed up.

"No hurry." I leaned in so the fall of his hair brushed my cheek. "This is precious," I whispered and to prove my patience I turned away, staring at the old show posters that papered the walls. Eye level to me, Louis Armstrong played at the end of some pier, a trumpet plugged into his mouth. His eyes bulged like someone performing at gunpoint. I looked back at Billy.

He parted the final folds of cloth, swallowing hard. Inside was a lump of stone, rough-hewn. I don't know much about stone. This was pale, slightly crumbly. Not glittery enough to be granite. Maybe sandstone or lime? The pale blond of Northumbrian beaches, of winter sun, dried grass and reeds. Fine powdered grains of it clung to the inner bindings of the cloth. Billy saw me stare at them and nodded approval. "Seeds," he explained, "that will furnish their land with forests for millennia to come."

"Wow." What else was there to say? It was a lump of stone.

He stroked it gently with his fingertip. "Hey there," he called to it, in the same spilling warmth with which he'd greeted me. "There she goes. You see her? You see her move? That ingenious press of muscle against the very grain that is her atmosphere? That is both of her and apart from her."

"Billy," I began, at a loss, but was saved by the waitress who placed a saucer on the table with the bill.

"We closin' now."

I pulled out my card.

"He show you his pet rock?" she asked as she tore my receipt. "Nice, innit? OK, Billy Boy? Doin' all right today?"

"I have to go," I told him and stood. If I ran now, hard, across Hungerford Bridge, I might make the 6:40. I moved to hug him goodbye but his head was bent, cooing at his maiden as he began the elaborate process of re-binding.

I bolted. Barged the café chairs, the folk outside gawping at posters from old music hall turns.

The crowds were thick on Charing Cross Road. I had to slow. There was some demonstration in Trafalgar Square. The police had closed the road by the Portrait Gallery, funneling people East towards Covent Garden. I dived down a backstreet but it was blocked off by builders' hoardings. Tried to retrace my steps but here was another dead end. A yet narrower alley fed me into the tiniest courtyard. Though I knew this stretch of the world so well, I was lost in some kernel of it. I found myself staring up at a patch of sun on the rising wall above me, where a shimmy of muscular hip pressed through its surface, twitched, then withdrew inside its plane of stone.