ideas for borders

I've been looking at ideas for the borders, problem is the border are rather large approx 450mm wide and 30m long. I've been looking at slate chippings which I've seen for about £120 for 1 ton bag, would 1 ton be enough, is that a good price? But the girlfriend thinks black slate would look better I can only find it in Blue, Plum or Green. Ii'mopen to ideas other then shingle, but I need something black and doesn't cost the earth, any ideas?

Edit: at a 2nd look the blue slate looks black or are my eyes deceving me? Also would I be better with 20mm or 40mm slate is this personal preference?

1 ton bag (if it really is a 1 ton bag as they vary) will just about cover it

you wont find black slate, its only in those colours

20mm will cover better and go further than the 40mm paddlestones

thanks thermo, I'll pop to the garden centre and see what blue looks like as it looks quite dark which should be good

OK, what kind of stone occurs naturally in your area. That is the one to go for as it will look right.