I been burning this season, primarily SBH and white oak. A few splits of elm in there but I gave most of it away. In the past I've burned tons of apple, lots of red oak, silver maple, mulberry, lots of ash, walnut, a little cherry, pine, poplar, and a little pin oak (I think that's all). I never get them unless I have elm.
Yeah, I've been getting them bigger than the one pictured, but they tend to break when I shovel them out.
This oak that I burn and where they form from was in a housing development so I wondered if the minerals were from the fertilizers or some kinda spray. Nothing else I burn seems to form them but I’m short on options for wood now.