One Wednesday morning in early 1852, a curious case came up at the Lambeth Street Police Court, opposite Westminster in London.
It concerned a schoolmaster, accused of obtaining money from a Kent Road pawnbroker by false pretences.
This gentleman had pawned a beautiful gem he'd obtained from one Dr Henry Jeanneret, recently returned from Flinders Island in Van Diemen's Land.
Dr Jeanneret was a highly-regarded dental surgeon, who'd been Superintendent of the Aboriginal establishment on Flinders Island through the 1840s.
He'd made a collection of the local stones, especially the exquisite topaz, often called Killiecrankie diamonds.
Dr Jeanneret had asked the defendant to sell some of the stones. When the defendant approached the pawnbroker with one of the larger specimens, both he and Dr Jeanneret believed it to be a diamond.
Subsequently, however, the pawnbroker had doubts and showed it to a lapidary, who pronounced it a fake.
A complaint of fraud was then laid against the hapless schoolmaster.
The defendant's solicitor made much of the fact that his client had given his real name and address to the complainant. Would a crook have done so?
Dr Jeanneret was called, and testified that he had believed the stones were diamonds - "as pure as the celebrated Koh-i-Noor".
He'd shown them to a lapidary himself, who pronounced them genuine. He then produced a box of the facetted stones, and they certainly looked the part. He handed them up to the magistrate.
The magistrate was doubtful, but impressed by their quality. Not having expertise in gems, he sent for a jeweller, who had a store near the court.
Shown a large Killiecrankie stone, the jeweller initially thought it couldn't be a diamond.
On trying to scratch it with his little jeweller's file, however, he found it far harder than expected. Faced with its brilliance and unexpected hardness, he finally said he could not swear it was not a diamond.
The solicitor then brought in several witnesses to attest to the defendant's high character.
The magistrate was in a bind. No one in the court now believed the beautiful Flinders Island stones were real diamonds, but the court's expert, a professional jeweller, hadn't been able to tell the difference.
He finally decided to admonish the defendant for not explaining the whole story to the pawnbroker, so as to allow him, with the full facts, to decide whether to accept them as real or not.
But with no one able to prove they weren't diamonds, he was not prepared to proceed to trial. The prisoner was discharged.
And so Killiecrankie diamonds had their day in court. They were so good, they could fool an expert, and they came in such beautiful colours - clear, yellow and pink.
Don't try to pass them off as diamonds today though. Lapidaries now have a better instrument, called a refractometer.