Prince fans may recognize Crystal Ball as the title of the aborted triple-album project that was ultimately released as the classic 1987 double-disc set Sign O' The Times. As is often the case with similarly prolific artists, from Eno to Zappa, much of that unreleased material wound up back in the vaults, but in recent years, Prince has been digging up rarities to placate fans, frustrate bootleggers, and thwart the efforts of his record companies to rein in his excesses. Anyone wondering where that extra record went need look no further than Prince's new rarities collection Crystal Ball, released on his own NPG imprint. Its predecessor, 1996's ballad-heavy Emancipation, was a three-CD, 180-minute celebration of Prince's release from the so-called "slavery" of his major-label contract; Crystal Ball is a similarly sprawling set that finds him reveling in his creative freedom. The collection holds hidden gems ranging from the infamous "Camille" tracks left off Sign O' The Times (featuring Prince's squeaky-voiced alter-ego) to songs obviously left over from the sessions that spawned his underrated 1995 album The Gold Experience. In fact, much of Crystal Ball's arcana tests the breadth of even the most avid Prince fan's knowledge, forcing listeners to take into account all of his many musical guises to guess which era produced each song. There are raw, uncut funk jams; psychedelic suites (like the sublime title track); new-jack rap; smooth soul; and rock-jazz hybridsall willfully eclectic proof of Prince's still-apparent genius. A note to confused consumers: The conventional retail version of the set, available exclusively at certain superstores, is packaged differently from the mail-order edition (an awkward, petri-dish-like approximation of a crystal ball). Ever the techno-savvy salesman, Prince makes the liner notes (and, for that matter, the song titles) available only on his web site (www.crystalballcd.newfunk.com), and ordering the set from the site nets you an additional disc, the enlightening acoustic album The Truth.