Scores of Scoria
I was thinking about my good fortune in having a garden that is somehow part of the Mattapan Volcanic Complex of the Roxbury Conglomerate. I dig up scores of Scoria, a mafic volcanic rock, and use it to border the garden and line drainage beds. Very pretty stuff. And volcanic soil is a fertile gold mine when it comes to soil. I am not sure I have such a green thumb as I do very good luck to have nutrient rich soil. Whatever I put in my teeny plot, I get it back in abundance! Lucky me.
Here's a link to topographical map of the Boston area, from a Geography blog called "Written in Stone...seen through my lense."