|The Diamond Authority killed Pink Diamond|
|Date of Publication||August 8th, 2016|
The reason why Pink Diamond is absent is because Yellow, Blue and/or White Diamond offed her in some way.
Throughout the show, the Diamond Authority's influence on earth is show, most notably their sigel. The modern symbol of the Diamond authority displays a set of triangles in yellow, blue and white colors. Older versions of this however are composed of rhombus shapes, with a pink one included. Other clues involving these symbols point to Pink Diamond's absence.
The first time Pink Diamond's existence is ever confirmed in the show was in the episode "Earthlings", where Jasper reveals not only the existence of Pink Diamond - Rose and Jasper being made for her specifically - but that Rose Quartz was in some way responsible for her absence. It is currently unknown in what state Pink Diamond is in, whether or not she was shattered, corrupted, bubbled or indisposed in some other way. In "Back to the Moon", Eyeball claimed to have seen Rose Quartz shatter Pink Diamond herself.
My theory is that Rose Quartz in-fact had nothing to do with Pink Diamond, and that it was her fellow diamonds that got rid of her, and then blamed/framed Rose Quartz as a means of legitimizing themselves for antagonizing the rebellion for their followers.
In real life, the only thing strong enough to scratch a diamond is another diamond, though it's quite easy to crack/shatter them with any object, as long as you apply enough force. It is also no surprise that the Diamonds would do this, as it is displayed that the Diamonds are entitled to themselves. Blue Diamond would have shattered Ruby for fusing with Sapphire while saving her and Yellow Diamond would destroy the Earth for the satisfaction of seeing it die. Gems everywhere follow them without question, looking to them as gods to be feared and worshiped. In "Monster Reunion", it is revealed that the Diamonds were the ones who corrupted nearly all of the gems on Earth, regardless on whether the gems there where fighting for or against the rebellion. With all of this, it would be no surprise that the Diamonds would go to such lengths as to kill, torture and/or imprison one of their own.
Why Pink Diamond specifically is not stated, but there are multiple reasons why. Given their petty nature, it is possible that Pink Diamond was simply at a difference of opinion to the others. It is also likely that Rose Quartz - confirmed by Jasper to have worked for her - inspired some sense of rebellion in Pink Diamond as she did for the rest of the Crystal Gems.
With the release of "The Trial" new evidence has come to light that while it might not have been the Diamond Authority as a whole, the idea that a Diamond was responsible has become more plausible.
According to the evidence presented by Green Zircon, Blue Zircon, Blue Diamond and Yellow Diamond, the alleged story was that Rose Quartz encountered Pink Diamond just outside of her palanquin and shattered her in front of her pearl, her Quartzes and various other eye-witnesses. However, it is made apparent that no actual alarm was sounded and that despite all of the protection, Rose Quartz (the only Rose Quartz not to have been captured and bubbled) somehow got past all of her security and caught Pink Diamond as she left the safety of her palanquin (something an inclusive Diamond would not do for just anyone after hundreds of years of wartime).
In "Bismuth", Bismuth confessed that Rose had her make her iconic sword to cut through a gem's physical form without harming the gemstone. Regardless on whether or not her sword is capable of cutting through a gemstone (much less a diamond), the detail that the sword - a weapon made not to shatter gems - was used to shatter Pink Diamond is a relevant detail.
As to which Diamond is responsible can be more tricky. Blue Diamond is unlikely, as she has spent her time since then mourning Pink Diamond's death to the point of neglecting her duties. She allowed the trial to go on without incidence and allowed Steven's side to make their case and was even legitimately shocked by Blue Zircon's assertions and evidence.
Yellow Diamond being the culprit is slightly more plausible, as she spent the entire trial trying to skip all formalities and protest Blue Zircon's allegations just to see Rose shattered and leave everything behind her. When Blue Zircon brought up the idea that is may have been her who had shattered Pink Diamond, Yellow Diamond reacted by poofing her and Green Zircon in a fit of anger, choosing to ignore the evidence and have Rose shattered anyway despite Blue Diamond's disagreements. This could mean one of two things, either 1) she was the one who shattered Pink Diamond and has been avoiding suspicion this entire time, or 2) Yellow Diamond is just naturally spiteful and prone to anger, as while she tried convincing Blue Diamond to forget and move on from what happened in "That Will Be All", she was clearly just as sad as Blue Diamond was.
As for White Diamond, not much information has been given about her personality or history, but she is implied to be the oldest of the Diamonds (given the high number of planets on her mural in contrast to the one planet and its moon on Pink Diamond's mural) so perhaps she possesses a higher step of authority over the other diamonds and may have had a plan that involved the youngest diamond's death.