It’s all true, and the incontrovertible proof has gone on display in the British Library. Side by side with original manuscripts and illustrations for the Harry Potter books, in an exhibition that opens on Friday and has already sold a record 30,000 tickets, there are dragons’ bones, a mermaid, a step-by-step illustration (on a scroll six metres long) of how to create a philosopher’s stone, a black crystal ball owned by a 20th-century witch known as Smelly Nelly, and a broomstick on which another west country witch regularly startled Dartmoor walkers.

Even JK Rowling, on a preview visit to the exhibition combining a history of magic with her creations, was astonished to come face to face with the tombstone of one of her characters. She tweeted the image, writing: “Guess what this is? I’ve just seen it and was mesmerised …”

The lead curator, Julian Harrison, explained that it was a memorial to the alchemist and suspected wizard Nicolas Flamel, who died in 1418 and whose grave, when opened centuries later, contained no trace of a body. His tombstone, borrowed from the Musée de Cluny in Paris, is said to have been rediscovered in a Parisian grocery in the 19th century, upside down and being used as a chopping block.

The curators have borrowed from national and international collections and from the archives of JK Rowling and her publishers, including her fanatically detailed plotting for the books, with grids of every character on every day. There is also a scrap of paper that proved as magical as any potion: the pencilled opinion of Alice Newton, the eight-year-old daughter of the founder of Bloomsbury, after eight other publishers had rejected Rowling’s proposal. Alice wrote: “The excitement in this book made me feel warm inside. I think it is possibly one of the best books an 8/9 year old could read.” The consequence was 400m books sold in 68 languages, and still rising daily.

There are several pages that never made it into the final version of the first Harry Potter book, where Ron and Harry were to have crashed the flying car into a mermaid-infested lake rather than the Whomping Willow. “Not as pretty as they look in books, are they, mermaids,” Ron remarks – which is certainly true of the neighbouring mermaid on loan from the British Museum, ingeniously stitched together centuries ago in Japan from a monkey and a fish.

The exhibition includes the oldest objects in the British Library: scorched, cracked bones used for fortune-telling in China more than 3,000 years ago, when they were widely believed to come from dragons. A book from the collection records a more recent sighting: a “monstrous dragon” that crash-landed in a field in Bologna, Italy, on 13 May 1572, and was on display in a local museum for a century after.

Visitors will be offered a chance to have their fortune read through a set of digital tarot cards. When the co-curator Tanya Kirk consulted them on the eve of the exhibition, she was told: “You’ve been working very hard towards a goal.” “So that was obviously completely convincing,” she said.

Kirk admits to “a very soft spot” for Smelly Nelly, a witch from Paignton who believed the spirits would be attracted to very, very strong perfumes. However, she admits to having been slightly spooked by a cauldron, thickly coated with an unpleasant black gunk, recovered from a Cornish beach after three local witches attempted to conjure up a demon but fled in terror when it exploded.

“Do I believe?” Harrison mused. “That’s not entirely a straightforward question. Take the question of potions, which we now see as magical but in medieval times were a recognised element of medicine. I would like to think I could do a little bit of magic, and sometimes I almost feel I can.”

• Harry Potter, a Journey Through the History of Magic, is at the British Library, London, 20 October – 28 February 2018, and the New York Historical Society from October 2018.
• Harry Potter: A History of Magic, BBC2, 28 October.