I know exactly what you mean I have had a couple of Yew logs where there has been 'heart' wood spreading right out to the bark and also wood which is somewhere between sap and heart wood.
My personal Yew ELB is made from a piece where immediately under the bark was darkish wood, then beneath that a paler strip and eventually real heart wood. I roughed off the outermost dark layer and got down to the slightly paler band and used this as the back.
See pic here:-
http://bowyersdiary.blogspot.co.uk/search?q=what+the The bow turned out fine. As it wasn't the best bit of Yew in the world I heat tempered the belly, it has little set and shoots fine.
This post shows a similar piece I cut recently, it was a decent sized log and yet has poorly defined sapwood, whereas the skinny churchyard yew next to it is gorgeous!
You can see the branch 'in the tree' and it looks like it should be perfect
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http://bowyersdiary.blogspot.co.uk/2012/11/variability-of-yew.htmlBottom line any Yew is better than none, and is likely to be better than many white woods.
Del
Out of interest we were shooting at a 160 yard makeshift 'clout' flag at the weekend, a lot of the guys found they couldn't reach it and moved down to the 120 yard flag.
My 50#Yew from compression wood made it no prob.
I'll bet if you'd asked those guys how far their bows shot before we started they'd have all said 180 plus...