I used some gravel from the big box store called Quickrete All-purpose Gravel. I believe its intended use is to mix into cement. But the size and shape are perfect for the gaps between our flagstone pavers. They're literally the size of peas, unlike "pea gravel" from the same big box store which ended up being the size of walnuts.

Because these quickcrete rocks are round, they don't stick to your feet; and though occasionally some do get up on top of a paver, the size is small enough that it's not a problem (unless maybe you're barefoot.)

This gravel is inexpensive; I believe they're 50 pound bags for around $4. Well, I just found a link - don't want to advertise this store, but for convenience' sake: http://www.homedepot.com/s/quickcrete%2520gravel?NCNI-5

You could use a polymeric sand joint filler. Check out this link. http://www.quikrete.com/productlines/SandPolymericJointing.asp

Good luck!

Thank you all for your advice. Appreciated !

I do not know how big the gaps in your flagstone are. Gator Dust makes a very nice polymeric sand as well that is formulated specifically for stone as opposed to pavers. http://www.alliancegator.com/products/polymeric-sand-and-dust/gator-dust/

It is really only supposed to be used for gaps up to 6".

Old post 2014.