Is this job lienable?
Gravel extraction would likely give rise to mechanics lien rights in Montana as long as the ultimate goal of the project wasn't to remove the gravel simply to make it available for sale. But, if the point of the project is to extract gravel so it can be sold, then mechanics lien rights would likely not be available.
This question sounds very similar to another one posted here in the Community - and you can find a detailed breakdown of the relevant Montana statutes here: Can I file a mechanics lien? | Belgrade, MT.