I finally got around to cleaning out my wife's desktop pebble water fountain. It has been abandoned for a long time and the water has evaporated. I thought the reservoir was filled wih little bits of straw, but upon closer examination, see that they are all little crystals. The water obviously spend a lot of time trickling over the pebbles, and sitting in its reservoir of slate, so I imagine it's dissolved some minerals from there. They're ~5mm long and .5 wide and of nearly uniform cross section end-to-end - few imperfections. I can't quite tell the angle of the faces, which is an important clue, right? They are opaque creamy white / very pale yellow. They don't look shiny, they look as if they'd feel like straw to the touch. What mineral do you suppose these crystals are?