Smelting trees?

Firewood is a supplemental heat source here,all supplied by the land I live on.It’s burned in a stove that yields a fine fluffy white ash byproduct.
However I note that wood from a certain area produces a metallic clinker (the sound of fire tools hitting it ) and closer examination reveals a reflective bluish green non-magnetic globule amid what looks much like slag.
The "certain area " is bottomland,where a spring fed stream runs.Since I process all the wood , I assure there is no metal therein (wreaks havoc on saw chain).Think tree house nails,land postings,etc.
Sometimes there is a prominent blue green stain on the wood when split open,not the black color of tannins reacting to ferrites,common in oaks.
What is causing this?

Well, the blue-green thing sounds suggestive of some copper compound, however, I think if it were present in any concentration enough to be visible, it would kill most trees.

I think it’s more likely that your blue-green globule thing is some sort of silicate glass.

Here is a study that contains the chemical composition of wood ash for various kinds of trees:

Rough averages yield 30% calcium, 10% potassium and 7% magnesium, mostly in the form of carbonates.

       I initially thought copper,too.The OP left out a factor,it being the property is virtually atop an area that was at one time the premier nickel mining area worldwide,or so the locals say.
   Whatever the substance is ,what is the means of absorption? Do trees precipitate whatever is dissolved in their water uptake ?

Many nickel salts are green too.

I use a stick magnet to pick up nails around construction sites before parking my crane. When i swish the magnet through ashes from a fire i get a lot of debris on it. Then i checked the camp fire at the lake and get lots of debris on the magnet.
There must be iron in the ash??

Yeah, but it could have come from a variety of different sources:
-Bottle caps and other stuff people threw into the fire
-Nails, wire, other ferrous items that were in the wood (especially if it was pallets or something like that)
-Possibly (but I think not likely under small fire conditions) ores from the surface of the ground, accidentally smelted by the fire (or maybe ores that were already magnetic, broken up by the heat into small enough pieces for the magnet to pick up).

According to my previous cite, an average of around one quarter of one percent. Not very much.