Flint is usually categorized as a form of quartz or crystalline silicon dioxide (see the URL below). It is a hard stone that will wear well but I know of no reason why you shouldn't use it in your pond. As to supporting the growth of algae....I highly doubt that it is worse than any other stone that we use. A couple of minor points: when flint breaks it will leave sharp edges - be careful that none of the edges ride on your liner or if so put a pad underneath the edge. Secondly, some folks like to make various decorations out of their stones by drilling or cutting them with diamond saws or special drill bits. When cutting this stone wear a respirator --- the dust IS harmful to you. You will not have to worry about flint wearing off or in anyway getting into your pond...it won't interact significantly with the water (pH, alkalinity, etc.) nor will it harm fish or plants. The big plus for this stone is that it won't break down with wear over time as some shales and softer stones will.
---David

Here is a link that might be useful: A bit of info on flint

Thank you David for your insight... I'll be better armed for the next company and in going back to deal with this company when he gives us a final estimate.

Elizabeth

David already cautioned you about the possible sharp edges on split slate cutting into or piercing the liner in your pond.
If you do have occasion to split any of the slate, it is easy to round the sharp edges off with a concrete cutting blade on an electric grinder. Chamfering the edges will help take away the risk of puncturing the liner.
And as David suggested, wear a mask while using the grinder on the slate.
"Horton"

Just a note of explanation: generally a respirator has a harness or at least four points of suspension to help it mold to the contours of your face. Those white masks with a rubber band which you see folks wearing to protect themselves....well, they're not that great for respiratory protection of your lungs. Oh they'll remove the "baseballs" and "basketballs" of the aerosol world but not the really fine stuff that penetrates into your lung. I use one that has a HEPA filter cartridge in it. Let your kids run around at Halloween with those white face masks.
---David