Are only things that were once alive...

Combustible?

Is there anything combustible that for absolutely certainty not a byproduct of life?

I don't mean "can it melt" or "can it glow when red hot", or "can it be burnt to a crisp in an oven".

What I mean is, can you put a match to it and it will catch on fire and burn by itself to ash without any further need to be heated.. anything at all that is something you can hold in your hand (not a gas)... that for sure was never a byproduct of something alive?

Yes, magnesium for one. Don't believe me? Go to a hardware store and buy some magnesium ribbon, all you need is a cigarette lighter to get it burning.

Uhh no... Methane existed long before life was around. Well as for something to hold in your hand... a leaf isn't a byproduct, plants make and use them. And a reaction that will continue on its own after being heated is Hydrogen and Oxygen to create water (not producing ashes of course since it doesn't contain carbon). Of course it can be debated that Oxygen was the byproduct from plants.

Well something else you can hold in your hand is pure sodium. Sure it might explode in your hand, or within the air depending on the humidity.

Are you asking this question since your wondering about organic compounds like octane/propane (Fossil Fuels)?

Get yourself a nice dog bone and test it.