Image imported from derpibooru 
Original Description:
 
Doorbell rings  
Rarity: “Oh, hello there dear! So sorry to say, but I’m tremendously busy right now trying to get my new Autumn Collection line of outfits done in time for the season; they are looking très magnifique if I do say so myself!”  
Rarity giggled to herself like a school-filly, proudly showcasing the sparkling gowns beside her.
 
Stranger: “No need to worry, Miss Rarity, this will only take a moment. I hear you’re quite familiar with gemstones?”
 
Rarity: “Oh yes indeed, but you might want to talk to Pinkie’s sister, Maud! She’s quite the accomplished geolo~~”
 
Stranger: “What gem is this, Rarity?”
 
Rarity’s eyes lit up upon seeing the bedazzling jewel thrust before her.  
Rarity: “Why, it’s a diamond, of course. And what a gorgeous diamond it is!”
 
Stranger: “No no, really look into it Rarity. What do you see?”
 
Rarity: “Wha? This is preposterous, it’s obviously a diamond! Look, I would love to sit and chat another time, but I’m quite occupied at the mo~~”
 
As Rarity looked deeper into the diamond, she could almost see a pattern in the glinting light. Pulsing, growing, alive.
 
She felt herself subtly let go of the tension building up within her. The view was just so… entrancing. Maybe her dresses could wait for a few moments after all. Her feverish work schedule had left her so drained that actually relaxing for the first time in weeks felt divine.
 
Stranger: “It’s beautiful isn’t it?”
 
Rarity: “Y-yes…”
 
Stranger: “Much like yourself, Rares.”
 
Rarity instinctively blushed at the compliment. Normally she would have been offended by such a strong approach to courtship, but she couldn’t seem to care in her current state. The world around her felt heavenly, as if she had just woken up on a lazy Sunday afternoon under a warm fluffy blanket with a mug of fresh hot cocoa.
 
“You see, this diamond is owned, Rarity,” the stranger spoke.  
The stranger’s articulation seemed… Odd.. The words punctured the air as if they were staccato notes, and the stranger’s emphasis on “owned” felt like a crashing chord barreling its way through Rarity’s hazy mind.
 
“Diamonds always look prettier in service to their master, do they not?” the stranger continued.
 
Rarity’s blush deepened, her subconscious fully aware of the lascivious innuendo the stranger had just uttered. She hated to admit it, but a diamond not in use was, by definition, a diamond wasted.
 
Stranger: “Focus on how good the gem is making you feel. Don’t hide your feelings. You know you love this.”
 
Rarity said nothing at the stranger’s resolute commands, instead biting down on her lower lip to hold back the increasingly flushed, needy sensations building up inside her. The world around her now seemed entirely vacant, her vision consumed by the sparkling, pulsing beauty mere inches away from her eyes.
 
Stranger: “It’s time for you to find your inner diamond, Rarity.”
 
Rarity’s eyelids drooped, her willpower faltering to the overwhelming bliss around and within her.  
“Y-yes…” Rarity spoke, as if in a dream.
 
Stranger: “Yes what?”
 
“Yes, master…” Rarity panted.
 
“Good girl,” the stranger said, casually producing a collar and leash from a nearby satchel.
 
“You’re going to love your new life,” the stranger commanded.