A black hole forms when the radius of a group of mass is smaller than the allowed limit... in which case matter collapes into other matter and everything becomes a point. This happens when the mass of an object is so great that the force that keeps neutrons from occupying the same space is out powered and the process of collapsing into a single point begins... This can also happen if two large (huge) forces act on either side of a group of matter forcing the radius below the threshhold and boom! you got a primordal black-hole.
This (according to hawkings) can happen to a group of matter with a mass less than 1 hundred-thousandth of a gram. These of cource only last for a short time because of Blackhole evaporation (i can explain... just ask) Even such a black hole with a mass of 1 million tons would only last 30 years. And 1 billion tons of mass would last 300 million years. According to his calculations a blackhole with with a mass of 4 billion tons would still be around today. He originally thought it was possible for us to have a couple primordal blackholes in our solar-system. this was proven wrong by the simple fact that we would be able to detect the gamma-ray radiation. But there is a good chance that they are out there... But unlike a regular blackhole that`s event horizon is miles across, a 1 billion ton primordal black hole would have an event horizon the size of a proton... so they hard to find.

I hope this helps
chuck