Diamonds are a Mare's Best Friend by CalculusFilly
Link - http://www.fimfiction.net/story/120120/1/diamonds-are-a-mares-best-friend/why-is-it-made-out-of-diamond

"Twilight goes to Rarity, seeking advice about what to when her normal toys aren't satisfying her. Strangely enough, Rarity thinks the answer lies within an unusual object: a diamond dildo! Twilight is perplexed by such an item, and takes it upon herself to study the science behind what would make such a thing so great."

Music used - Diamonds Are Forever (1971) by Don Black, performed by Shirley Bassey