The magic cue ball is an item in Homestuck. Unlike a classic Magic 8-Ball, it always reveals the truth to its owner and makes predictions with alarming precision and specificity. Unfortunately, and also unlike a Magic 8-Ball, it lacks a portal on its surface that allows you to view the prediction. Because of its opaque surface, only those with some sort of can receive answers from the item. In Hivebent, it is revealed that Vriska Serket was in possession of a Magic Cue Ball in the past and was able to receive answers from it to help her strategize against her opponents in Flarp. Her VISION EIGHT-FOLD could see through the surface of the ball, revealing the prediction concealed within.
It is implied in the diary of Mindfang, Vriska's ancestor, that she and the Expatri8 were the previous owners of this ball, with Doc Scratch having given it to Mindfang as her "mentor", and Mindfang having given to the Expatri8 due to his void powers protecting it from Scratch's omniscience. In addition, although the cue ball always gave her accurate information, it was always phrased selectively in a way which impacted her negatively. As part of the troll's chain of revenge, for killing Aradia, Terezi revealed to Doc Scratch that Vriska was in possession of it, causing him to lose his characteristic cool and detonate it in her face, costing her an arm and her vision eightfold. In return, she performed a double psychic reach-around and forced Terezi to stare at the Alternian sun, blinding her.
The only other known Magic Cue Ball is owned by Jade. seems like a cheap toy, though since this is Jade it is definitely the real deal. It was a gift from her Grandpa along with the rest of her fortune-telling knick-knacks. A copy of the ball is also present in Jade's dream room on the moon of Prospit, and it's implied he may have taken said copy back in time to Earth with him to become the original.
A consort ends up with the ball after Jade enters the medium and gives it to Rose. Doc Scratch indirectly tells Rose that she is able to see the responses by means of her powers as the Seer of Light. According to Doc Scratch, the Magic Cue Ball is one of his 'seeds'. It is also revealed that the cueball was half of Doc Scratch's ectobiological makeup, and that instances of himself have spawned in countless universe, implying that perhaps any first guardian whose ectobiological makeup includes a magic cue ball becomes an iteration of Doc Scratch.
In the B2 session, after the planets are moved into orbit around Skaia, another consort brings the cueball to Dave. Jade then combines the cue-ball with Dave's Deringer to create Caledfwlch. According to Jade, the cueball is in fact the only weakness of Doc Scratch's master, Lord English. Two other cueballs have also been seen, although it is unknown if they have the same abilities. One in Doc Scratch's grandfather clock, which broke along with Doc Scratch's cueball head when Lord English entered the universe, and another in Doc Scratch's magnum, which Spades Slick uses to kill Snowman. Andrew Hussie later tries to use the gun to kill Lord English, but laments that the gun only had one cueball bullet in it, implying the cueball Spades Slick used was in fact dangerous to Lord English. Meenah also finds a memory of the cueball in a dream bubble, and wonders if it is a juju. Lastly, Caliborn's quest to destroy the twelve planets of his session requires the well-timed detonation of bombs that resemble cue balls, which he finds at the core of each planet after having slain a powerful underling of Yaldabaoth.
As with Doc Scratch's clock (which may well be the same clock), the pendulum of the clock arbitrating Caliborn's immortality – which he gains access to after defeating Yaldabaoth in [S] Collide, and destroys in [S] Act 7 – is also tipped with a cueball. This could possibly be the origin of the magic cueball, as well as the reason it is English's weakness: as a component of the clock, it might hold influence over his immortality.