comment: >
  
  Trees are made of organic compounds and there's literally millions or billiona of alternatives to that compound. If by tree you mean trees resembling earth trees they will be extremely rare but it you mean anything which performs photosynthesis then it will be more common than steel
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  I guess it depends on how big your list is. Do you mean “one of” the top 5 rarest? “One of” the top 100 rarest? There are plenty of building materials rarer than wood. >
  
  I'm guessing most are easier to make outside the Earth than wood >
  
  Not really. Most modern building materials are manmade so they don’t exist outside Earth either. >
  
  But they are mostly made out of stuff you can mine, right? You can bring a mining rig to another planet and just process the resources there to create other materials, but you will likely not be able to grow trees >
  
  You could theoretically enrich the soil on another planet, create an enclosed habitat with the correct atmospheric composition, and grow trees on another planet.
  But we’re not talking about theoretical situations. Right now, there is no wood on other planets. There is also no steel. There is also no concrete. There is also no vinyl. Etc. >
  
  But it would be easier to create steel than create wood so we can assume wood is rarer than steel >
  
  No, we can’t. That’s not how rarity works. The rarity of a material depends on how much currently exists. Not how much could potentially exist. >
  
  We don't know how much exists. So we can't conclude that steel is rarer. But if we take it based on how much could potentially exist then steel is more common. If you lack the complete data you have to make the closest guesses >
  
  We do know how much exists because we created all of it. Steel requires intelligent life. Trees do not. Even of we assumed an non-zero number of life harboring planets other than Earth, trees would be more common than other alien cultures inventing steel.