I have a couple of pieces of jewelery made from the volcanic ash from Mount St Helen's. It's essentially posh glass - obsidian - and is a very clear dark green, similar to a Tsavorite garnet. It sometimes comes in red too. The stone is called Mount St Helens or Helenite.
We lived outside of Seattle when the eruption happened. My dad took me fishing that morning. We heard a loud bang, and my dad told me that someone must be shooting cannons. I had another couple hours with a fishing pole to daydream about pirates and cannons and the like. When we got home, my mom told us what had happened, and it was even more exciting (and scary!) than pirates.
The next day was a Monday, and school was canceled! It was like a snow day, but better! Some other kids and I walked up to the top of the hill to see the "mushroom cloud." I was really disappointed that it didn't look anything like a mushroom. We couldn't stay outside for long, though, because everything was covered in ash and our moms made us come in. I remember asking one of the older kids what would happen if I breathed too much ash. "It'll cut your lungs," she told me. That's when it was no longer cool, but scary. I tried to breathe shallowly for the rest of the day.
My folks still have a couple jars of ash in a closet somewhere that we collected off of the car and the bushes out front. My most vivid memory of the whole thing (possibly because, if I remember correctly, my dad took a picture) is of ash covering the rosebuds in the front yard.
I have ash and rock from Mt. St. Helens, collected during a visit 8 or 9 years ago. I remember seeing a road crew trying to bulldoze some space for a new lane or something--every time they'd cut into the embankment, it'd just collapse in a cloud of fine ash.
I've also collected samples from Guatemala (pumice stones floating in Lake Atitlan, looking just like styrofoam) and from Kilauea (chipped right off the cooled flow).