I tried them.
The tufa failed completely to set up.
And the stench, OMG, the smell was horrible.

That said if you used very course coffee well soaked to remove the acids it might work but the stuff from Starbucks was terrible.

Did I mention the stench?:)

I don't think ours stunk, but it just wasn't very solid or satisfying....would not try this again........

Vicki

what about as a dye?

grasshoppersilks

yes, people tried it, there was discussion about the use last year. Maybe you can find it. As I remember, nobody was very happy with it. Maybe the oil in the coffe grounds was a factor too. The best organic material for Tufa is still the peat moss.