A mib equals a handmade, art glass-marble (or glass marble in general, but I only include an artistically fit, contemporary marble for my definition).
As of autumn 2006, amazing artists, such as KENAN TIEMEYER, DREW FRITTS, BRAD BRANDOLINO (the Matrix Man), KEVIN O'GRADY, DAVID SALAZAR, etceteras, expand the possibilities to a frenzied degree. Of note, the fume technique is causing havoc among addicts, at the moment. Bidding war usually occur on Ebay and other sites. Uranium inclusion is always a welcome diversion. And optical is rare, and I am not talking about vortex, either. A huge marble, 2.60 inches in diameter or more, is a must. Personally, I stay away from a drab, puny mib, something that is less than 1.30 inches in diameter, especially with morose colors. And unintentional bubbles in the design vex most marble collectors, and these are usually a sign of carelessness in one's work, or even lacking in skills -- an amateur, perhaps?