@Squirmy Hooves  
I have a friend who’s just over 50kg clothed and about 185cm (6’1”)

Using hands for measurements? Well…

Unless the Equestrian karet is much larger, due to gems being worthless.

Well, it is called my “little” pony.

Faved, but not sure if accepted.

@zeldazackman  
Well, yes, but i am 1.51 cm\5’11”, so it is not extremely underweight. And since i am very smart and i know all the Pokèmon names and types, my brain also takes up another 6kg, and i have so much hair, that i think it may take 6kg too. (No wonder why i slouch a little…) So that is actually 46kg. So i’m actually pretty average-weight, it just looks less because of no physical training.

@Squirmy Hooves  
Holy crap, thats light weight, thats like “Pick you up and let you ride on my shoulders everywhere” kind of light

Actually, carats are a measurement of weight. The fire ruby is not the same thing as an Earth ruby, and some gemstones can be pretty light anyway, that ruby seems to be 5 or 6 inches in diameter, as the top of a pony s’ head touches just under a 12-year-old girl (like Blythe or Megan) s’ breasts. Compared to a grown-up tall woman (i.e. taller than 5’9”) the top of the ponies’ heads are just above the woman s’ hips. But not all women are that tall, a 4’7” tall woman would be closer to the height of the 12 year-olds. As the fire-ruby is a magical ruby, and also could be hollow, but even if it was hollow, it would still weigh, like 1kg or 2kg. If the base for weight is the kilogram and not the gram in their world, a 20 karat stone would actually be 4kg. Spike can still lift it, but he has average musculature. I am still kinda weak for my age (16), i weigh 34kg at most, and i’m no stronger than most 12-year-olds.

Makes sense, alot of it. No wonder you can’t see any “visible genitalia” on those creatures.
 
(only industrial diamonds have an actual purpose and benefit)

@Background Pony Number 17  
That makes some sense, as their society already has a lot of puns in it, and a carrot would be something every pony would be able to compare stuff to. I doubt that ponies have strictly established units of measurement outside of academia and bureaucracy.

Maybe Rarity didn’t say “carats”, she said “carrots”? That thing looks like it weighs about 20 of them.

Nevermind. I am their god now.

How am I supposed to tickle a pony with my house-sized hands!?

Prehaps Rarity is just really good at sarcasm.

@ExtremeLurkerHomeEdition  
Pffft, you’re just jealous that my headcanon became canon and yours didn’t. Neener neener neener!

@Splinter17  
Because the show staff are always serious and truthful on Twitter. Mhmm.

Larson has stated that this pic is accurate and completely canon!  
https://twitter.com/M_A_Larson  
Fourth tweet down at this time of writing.

Nah, as far as the mesurments of worth go you would be assuming that 20 karots for the is the same for us.
 
And as far as predicting what the relative worth of gems would be over there I would agree with the others who have already pointed out gems are pretty common over there. Many headcannons say that magic helps grow them and Pinkies family farm is one such area where earth magics are harnesed to cultivate them.
 
But that being said they would not be worthless, especially since they likely hold many magical uses and also are “consumable” goods and thus may be eliminated from circulation rather then just transfering.
 
It would be like giant lumps of copper, “dead common” but always in demand.

@gittonsxv Well, that’s a recent ‘innovation’ though, before that minted coins did have value equiv to what they were made from, and paper was basically an unstated contract that the local gov would reimburse you that much in weighted coin for exchanging it.  
Since Equestria has so many innate magical bonuses over many other civilisations bits are probably like the Florins of their world.

@Background Pony  
When was a pony onscreen with a human?

@Prof.NightJack
 
Then by weight, on a dragon scale.

@ALIS  
A carat is 200 milligrams(0.007 ounces for ‘Muricans), and as far as I’m concerned, 200mg on Earth is 200 mg in cartoons.
 
All silly headcanon aside, my guess is that the writers simply got “carats-as-purity” and “carats-as-mass” definitions mixed up.

@ALIS  
It’s not measured by size, it’s a unit of mass of 200 mg, it’s by weight

And yet ponies are about the size of a large dog compared to EQG characters…

Perhaps Gems are measured by Dragon hands seeing as how big they are. That human hand photoshopped in matches about the size of an adult Dragons hand.