IF it's done correctly it an be beautiful Spalt. Poplar and maple are the prettiest. Some oaks will with ???? results. the other 2 are more consistant.....and YES as one posted IF the timing is wrong = ROT !!! .....and I've done that also. This would be a long explanation to detail it BUT in a nutshell spalting is a rot process that starts with a bacteria/spores/fungi??? (not positive of correct one) BUT it's in 3 stages....as the 1st one is finishing up a 2nd one starts and there's a battle which creates the black lines...there's a guessing game as when to stop and saw due to waiting TOO long it starts stage 3 and starts softening and rotting kinda simontaneously ...the softening can be pretty BUT it requires extra work to stabilize. I just pitch some out in oak due to I waited too long.
Some like this style while others hate it...I myself have been a little more consistant on the correct time stage to saw BUT I have thrown almost full stacks away due to higher moisture and timing.
Check my website for spalted poplar and maple along with a few others as buckeye, ailanthus (tree of heaven/paradise....stinks like la..pew BUT that smell goes away when dry). Most pics are under heading "the Mill" under "New and Hot off the mill" With several slideshows of the differ styles and types.
Do a search on spalted maple and spalted poplar in the upper right corner....I've probably got some older threads with pics in archives.