What is a durable string to use on a marimba-any special material or maximum/minimum thickness?

Well if you're worried just about a durable string you could use anything all the way down to weedwhacker line. Another alternative is 1/8th inch climbing rope (obviously you don't climb with 1/8th inch rope but it's the same style rope that you climb with) One final idea that is incredibly hard to string but fairly durable also is plastic clothesline (it's got several strands of plastic inside a plastic coating...kind of like an elctrical wire)
These are just some of the things we've used before.

Parachute cord from the Army surplus store is another alternative.

Stay away fron any plastic cord.It will just buzz you to death. Parachute cord does work well and is very durable. I can supply it for $5.00. But, as mentioned, you can also get it from outdoor stores, Army Surplus stores, etc.

Bill Youhass
Fall Creek Marimbas
www.marimbas.com