Diamonds aren't stable and will eventually, over billions of years, decompose from their cubic molecular structure to carbon's more stable form, graphite, which has a hexagonal molecular structure.

Oh, here's another good gemstone related one!

Amethyst and citrine are both quartz varieties, and if the color source happens to be from traces of iron in the crystal lattice, one can be turned into the other. Heating amethyst can make citrine, and irradiating citrine can turn it into amethyst. This is because the only actual difference between the two is the valiance level of a specific election in the iron atom giving the stone its color.

Bedsheet thread counts have been artificially inflated for years by the shifty linen companies counting individual fibers that the threads consist of as threads themselves. It’s become a meaningless number, since there is zero regulation. If you want a nice thick heavy cloth, GSM is the number you want, but most companies won’t share this (looking at you, The Company Store) because they obviously don’t want you to know how thin and flimsy their products really are before you buy them.

You know how geese fly in a “v” shaped pattern in the sky? One side of the “v” is usually longer than the other. The reason for that is that there’s more geese on that side.

Several popular graphing calculators from Texas Instruments, including the TI-83 and TI-84, have a display resolution of 96*64, but only 95*63 pixels are used for graphing.

However, the earlier TI-81 did use all 96*64 pixels. The rationale for this change was to establish a central row and column for the axes and a central pixel for the origin. The cursor could only move pixel-by-pixel, and since the axes and origin would end up "between" pixels on the TI-81, they were inaccessible by the cursor.

There are more hydrogen atom in a single molecule of water than there are star in the entire solar system.

I just touched my nose. Until I posted this, I was the only person who knew this fact.

Most male cats, when investigating something with a paw, will use the left paw.

If you catch a frog in between your hands and quickly flip it around, you can get the frog into a kind of paralyzed state called 'tonic immobility'.

Here is a photo from Wikipedia:

OK, well, many years ago I was very interested in this phenomenon and decided to look into the literature.

I found a paper from 1928 titled "On The Mechanism of Tonic Immobility in Vertebrates" written by Hudson Hoagland (PDF link).

In this paper, the author describes contraptions he used to analyze the small movement (or lack of movement) in animals while in this state. They look kind of like torture devices:

OK, but, that's still not it.... The obscure fact is found in the first footnote of that paper, on page #2:

Apparently this or a similar effect can be observed in humans too?! In this paper, the author himself claims to have done this and that it works! I tried to locate more recent resources describing this phenomenon in humans but I could not find them... Is this actually possible? If so, why is this not better documented? Or, maybe it is better documented but understood as a different type of reflex today? Not sure.

the roslagsbanan commuter rail is the only actively used 2 ft 11 ^3^⁄~32~ in railway in the world.

...honestly, with a wikipedia article that extensive it hardly qualifies as "obscure".

Opossums have 13 nipples

The dot above the letter i is called a tittle.

Chinese scientists worked to create the "humanzee," a human-chimpanzee hybrid in the '60s. Female chimpanzees were impregnated with human sperm. The experiment was cut short by the Cultural Revolution - the scientists were sent to labor camps and a three-months pregnant chimpanzee died of neglect. The Soviets attempted a similar program in the '20s.

Ancient Egypt was ancient before it ended. The time when Cleopatra ruled is about as close to today as it was to the first pyramids.

A lot of people know of the April Fools 418 I’m a Teapot error code, but did you know there’s a full Hyper Text Coffee Pot Control Protocol from the same RFC for running a coffee pot server? It even includes an HTCPCP method named “WHEN” to let the pot know it has poured enough cream.

There are no extant recordings of George Orwell's voice

Living at high altitude for long periods of time can cause a disease that is otherwise most associated with cocaine and meth.

Pulmonary Arterial Hypertension has some weird causes, but it seems that high altitude and having to work for enough oxygen can cause the body to revolt.

The tiniest park in the USA is located on a city street corner in Portland, Oregon. Mill Ends Park

Edit: I fell down a rabbit hole. Corrected myself having posted it originally as “world’s smallest park” which is how I knew it - apparently it carried that distinction until Feb this year when a tiny space on a Japanese street (which was created in 1988) formally applied for, and was awarded the Guinness book of records title of World’s Smallest Park.

Also this one just popped into my head - the Guinness Book of Records was originally conceived as a means of settling arguments by compiling factual “records”. The original argument related to a shooting trip in England in which the Managing Director of Guinness Breweries partook, where a missed shot led to a disagreement about the fastest game bird. The realisation that arguments such as this would be commonplace, and that no resource existed to settle such matters - the niche for capturing these types of facts was identified and the book was born.

The entomology book Life on a Little Known Planet taught me that bebugs mate via "traumatic insemination". The female has no opening, so the male pierces the exoskeleton and the wound later heals over -- all of which allows entomologoists to count the number of times a female has mated by the number of scars on their abdomen.

When a drug company in the 80s scaled up production they accidentally created seed crystals that spread around the entire Earth's atmosphere that prevented other companies from manufacturing a generic drug without it attaching to the seeds and converting to the patented drug.

https://en.wikipedia.org/wiki/Disappearing_polymorph#Paroxetine_hydrochloride

HD-DVD and Blu-ray weren't the only HD video disc formats competing for dominance in the '00s. HD VMD which was basically a DVD containing more layers unsuccessfully tried to compete with the two. The company who produced it dissolved in 2008 and only a few titles were ever released on the format.

2” x 4” construction timber is 1.5” x 3.5” because of industrialisation (not shrinkflation)

Fun fact, you can, in fact, make sourdough with the yeast from a yeast infection, and bake with it.

Diabetics piss has so much sugar in it that you can make high end whiskey with it.

Not obscure but apparently a lot of people aren't aware that sheep don't have top teeth in front.

Darwin drank tortoise piss and, according to his documentation, didn’t hate it.

Barnacles, relative to body size, have the longest penises in the world

The Latin meaning of the color ultramarine is "over the sea" Also, they once made a pigment called mummy, which was literally made out of finely grinded mummy.

The smallest extant flightless bird in the world is the Inaccessible Rail.

There is a dust layer in the ice at the South Pole about 2km under the surface that interferes with about 5000 photomultiplier tubes spread out over a cubic kilometer in the ice that are watching for light created from high energy muons moving faster than the speed of light in the ice that were in turn the result of the very rare chance of a high energy neutrino interacting with the nucleus of a single atom in the ice.

A large amount of visual inspections on the inside of nuclear reactors is done literally with a camera duct taped to either a really really long pole assembled in sections or a rope. Operators "swim" the cameras to various locations and camera handling is basically an occupation in that field. You also need camera shots for any work being done on the inside of the flooded reactor with, again, really really long poles that end up acting more like pool noodles at such a length. It is silly and difficult work. Also you basically are wearing a trash bag sitting above a hot tub while doing this work. So it is a wild experience.

Not that obscure, but most orange cats are male since the orange gene is tied to the x-chromosom, so male cats only need one copy to appear orange. Female cats have to have the orange gene on both x-chromosom to be fully orange.

And usually only female cats can be calico/tortoise since the orange gene is co-dominant, so if they only have one copy of the orange gene both the orange and black will be visible.

A seemingly male calico/tortoise cat is usually intersex and sterile.