Purple Slurry = Blue Belgian. Correct? What color slurry does slate make?

I found a 6"x1.5" yellow coticule at the local flea market a few weekends ago. (My first stone.) I thought the backing was regular slate. (It was glued to the yellow.) Yesterday I read about the purple slurry, so I tried rubbing the stone with a small piece of 1000-grit sandpaper. The slurry came quick and purple. In less than three seconds, I had a nice purple slurry.

So is this a definitive test?

Thanks,
Christopher