DM: As for Pinkie Pie–
Pinkie Pie: I was a rock farmer once!
Applejack: Ah say again: What.
Pinkie Pie: But I wasn't very happy as a rock farmer... so I left home and went into baking and party planning! And now I'm an apple farmer! So it's going about as well as you'd expect!
DM: ...What ARE we supposed to expect here?
Rainbow Dash: *Rock*... "farmer"...?
Pinkie Pie: To harvest rocks!
DM: Okay, context: Her family owns and operates a gemstone quarry.
Pinkie Pie: Yeah! Rock farmers!
Applejack: Ah'm just gonna quit this line of questionin' while we're ahead.
I mean, when you consider that Pinkie Pie comes from a lifestyle where farming rocks makes sense... It puts a lot of her other craziness into perspective.
Pegasi alter the weather by kicking clouds. Alicorns move the sun and moon. I have no idea why so many people find the idea of farming rocks to be unbelievable.
Well yeah, and in a world where magic is a thing, it makes sense that gemstones and/or specific varieties of stone could be grown/harvested. Pinkie's family just takes the 'earth' in 'earth pony' a little more literally than most.
I mean, it's not like you can't grow gemstones in the real world. Heck, there's a rather large industry involved in doing just that! And that's without getting into the _silicon_ growing industry for computer chips!
Even naturally, crystals grow. Just usually very, very slowly.
If the quarry can be coaxed to manufacture new gems over the course of several years (or faster), rather than geologic time, that counts as "farming" to me.
Rarity may look at Pinkie in a whole new light if she's a source of gems.
I guess that, until now, Pinkie's backstory was just taken for granite.
There's an aspect to this that a lot of fandom doesn't get, because why would they: if you leave a field empty for a season, gradually the soil settles and medium-large rocks "float" to the top. Then when you want to work the field the next season, you have to go through and chuck out all the rocks so they don't screw up the equipment. This is facetiously called "rock farming." The whole gemstone thing I'm pretty sure was made up by fandom and either never outright confirmed or retroactively adopted by canon. It's a farming joke.
Source: my dad grew up on a farm, told me this and was confused when I thought it was hilarious.
I was today years old when I learned that "rock farming" is an actual thing. Now ask your dad if he's ever eaten a rock. And no, sand, dirt, ice, and salt don't count.
Though I would add that, at the very least, the gemstone thing did get added into the show. There's a big gemstone cave on their property, and there was that time where Marble and Limestone cracked open a geode as a metaphor for Maud's boyfriend.
The mechanism by which the rocks 'grow' is frost heaving--the ground freezes in winter and pushes the rocks up since that's the path of least resistance. Naturally, this is more of a thing in colder and wetter climates, since warm doesn't make ice and dry doesn't supply ice.
This is the first time I've ever heard of Pinkie's family having a Gemstone Quarry. I know in the show they actually just farmed rocks. Though I've only watched up 'til mid season 5.
To be fair, I used to live down the street from a dirt store, so this doesn't seem all that crazy to me. Looking back, I guess they were a garden supply store or something like it, selling different kinds of dirt to people whose local dirt might've been bad for crops, but at the time my mind was just blown by the idea of a business that could literally just sell dirt to people.
You also sometimes need extra dirt for landscaping purposes - to fill holes, make hills, etc. Just last weekend, I bought some bags of topsoil (for fill) & leveling sand to raise up & re-level some paving stones that had settled too much.
I used to work for a construction inspection company, and we had a project for a school district. As is often the case with schools, they made a custom specification for the soil that would fill all of their parking lot islands and similar planters. Rather than just specifying chemical makeup, they decided to require a specific grain-size distribution as well.
Cut to me showing up at the dirt store, asking for a sample to test for compliance.
"Oh, of course, we have samples right here." They point to these little berry basket sized cups of soil.
"No, I need three 5-gallon buckets, full to the top."
"...what."
Apparently potting soil isn't usually subjected to the full battery of structural aggregate tests. It wasn't my place to ask, but I wondered if they were really going to be able to supply several dozen dump trucks full of the stuff.
My explanation to myself for the Pie's rock farm used to be that they actually distribute rocks on regularly flooded fields, on which, when the water recedes, encrustations of salts and minerals remain. So, in other words, they would actually be salt farmers, trading in licking stones. I mean, equines love that stuff.
Don't forget that not only does the Pie family harvest rocks, they eat them too. Apparently some earth ponies are part dragon or something crazy like that. :3
One entry in the Infinite Loops stories has Discord check it out and declare it to be a draconequis egg¹, which would go a long way towards explaining Pinkie.
1: At least for that particular loop. Little things tend to vary, like home layouts, family histories or who rules Equestria.