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kt66

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Funkfingers

Could be sap crystallising.

Could be fungus, mould or rot. (The facts that the lines are more or less book-matched and that you have not mentioned any smells suggest that this is not so.)

VimFuego

I always assumed (with no evidence or expertise in the matter) that they were stress lines, be interesting to see what they really are

RandallFlagg

Aliens? 

stickyfiddle

RandallFlagg said:

Aliens? 

ICBM

It's natural colouration in the wood - if it was anything else it wouldn't be the same on both sides. If it's not always been like that, it's just something that's become more obvious as the wood has darkened due to light exposure.

Jalapeno

But do keep it out of direct sunlight, which wouldn't help.

AliGorie

natural - like you 'n me - hug that tree

anthonyw

What you are seeing are sap streaks.  They are toward the inner core of the spruce tree and are always lighty coloured.  As your guitar gets older, the outside layers will darken but the sap streak will remain whiter. The dark lines are a place where future cracks could develop as it's a punky bit of grain between the sap and the hard grain outside , but not to worry.  It's wood.

kt66

thanks