Artist's description:

"And the fifth entry in the "How They Met" Series is here: how Pinkie Pie's parents met! Complete with some lore I thought up!

To keep it sweet and simple, here's the context of the image:

Amongst the earth ponies who inhabit the rocky fields of Western Equestria, a tradition as old as time begins with the Pairing Stone giving off its lovely pink glow, signaling that the time has come for two ponies to marry. In this instance, Igneous Rock Pie and Cloudy Quartz were two such ponies who'd recently come of age and were instructed to find the "Stone that Matches the Eye" for their significant other. Now, as the story goes, every pony who was of age would go into the Crystal Caverns and become led to a certain gemstone that would link them forever to their partner and then, at the Pairing Ceremony that same night, they would match their stones to their partners and go off to get married. Luckily for Igneous and Cloudy, despite never having met before, they chose stones that matched them together and, meeting for the first time in front of the Pairing Stone, they immediately fell in love.

Their relationship after that was a bit rocky (no pun intended). They'd get into their fair share of squabbles and whatnot, but through it all, they had their matching stones which reminded them of their destiny to be together forever. It was an earth pony's way of using magic and with each quarrel, their love and bond only grew. Four daughters later and they are more filled with love now than ever before!"