1. The cursed emeralds of La Spezia. The story of the cursed emeralds begins in 1830, with the return of teenaged Bostonian adventurer-poet Hiram Johnson from Italy. Hiram refused to give details of his adventures, let it be widely known that he had suffered some dreadful trauma, and subsequently released a series of increasingly fevered poems about emeralds. Although the actual details were obscured by florid language, this served only to stoke up interest in the hidden treasure and its mysterious curse. Before long, a stream of would-be adventurers were making the trip to Italy. One by one, they returned empty-handed, refusing to say what they had seen. This state of affairs lasted some seventy years, until one Theodora P. Blunderbuss, a writer, decided to make the pilgrimage herself. She followed the coded directions of the poems, which led swiftly to a small house by the seashore, inhabited by an old woman with curiously green eyes. One learning her mission, she gave an extravagant eye-roll and informed her that all the adventurers had been taken in by a series of poems in the ‘why can’t I get laid’ tradition, stemming from the occasion, seventy years distant, when she had turned down Hiram’s propositions and brained him with a frying pan. The emeralds were none other than her eyes, the vast and brazen force protecting them the pan, and the identity of the poem’s damp forbidden caves was pretty obvious when you think about it. Since then, she had run a profitable business selling green glass souvenirs to visiting Americans, and so had been unwilling to reveal the secret until retirement.
2. Feldmann’s Contrary Bezoar. Believed to have been discovered in the stomach of a goat with two heads, Feldmann’s Contrary Bezoar was a dull grey stone with two rumoured properties. First, when turned the right way up, it was said to have the power to cure cancer. Second, when turned upside-down, it was said to have the power to cause cancer. Unfortunately, the difference between the two sides was rather difficult to discern. In fact, the bezoar was distressingly spherical and nobody really felt like finding out what would happen if one turned it on its side. As a result, its supposed curative powers were rarely invoked and it languished in a box in someone’s shed for years after its initial discovery. These days it is rumoured to have been sold on to a group of scientists in the pay of certain scurrilous newspapers, for the purposes of headline generation.
3. The Star of the Southern Ocean. This large star sapphire was originally discovered in 1892 in Queensland and subsequently displayed in the Australian Museum in Sydney for some years. On the night of June 27th 1950, however, it was stolen by notorious jewel thief Gideon 'Fingers’ Blackthorne. The full details of his cunning plan have never been revealed, but we do know that he smuggled the jewel out of the building up his bottom whilst dressed as a member of the cleaning staff. Scarcely had he had time to admire his new possession when a massive police search was launched. Back went the gem, and on went the next disguise. Over the next few months Gideon masqueraded as, respectively, a nun with a sapphire up her bottom; a fortune teller with a sapphire up his bottom; a chef with a sapphire up his bottom; and a well-known opera singer with a sapphire up her bottom. Eventually the search was scaled down. By this time, however, Gideon had discovered that he quite liked having a sapphire up his bottom. He continued to store it there, on and off, for the rest of his life, disclosing its location only in a secret will distributed to his heirs shortly before death and marked 'read BEFORE cremation!!!’. The Star of the Southern Ocean was returned to its rightful owners, who were oddly loath to have it back. It has not yet been returned to public display. There are those who say the stone remains curiously warm, with a mild pungency that cannot quite be erased.
4. The Stone of Ultimate Power. It is a little-known fact that, some years ago, there existed a diamond of exquisite purity, save that it was seeded throughout with atoms of an element hitherto unknown in our paltry human dimension. Besides giving the Stone an eerie green glow, these atoms allowed it to align the vast energies of the Void of Space and Time and channel them into an easily controllable and near-infinite power source. Thus the Stone was possessed of great power for good or for evil. The stage was set for a giant, all-encompassing struggle for the future of mankind. Before this could take place, however, the Stone was purchased by a French lapidiarist, cut into multiple pieces, and set in a matching necklace and earrings. The setting was quite beautiful and commanded a record price at auction; nevertheless, it also completely erased any world-changing properties the gems had once possessed. The remnants of the Stone are believed lost following a bomb blast in WWII Paris, which is probably for the best.
I’m pretty sure that these are all drops in Diablo III.