OK. Words.
Where is the line drawn? Rocks that have imprints on them vs. rocks that look really cool? It was determined that that my example was not in a grey area, so I'm looking to understand what areas are in the grey.
I think it's fair to say that not all fossils are created equal. I think you'd be hard pressed to take a fossilized leaf and trade it for the fossilized skeleton of a T-Rex. Some things more valued due to their rarity and demand. The infractions for taking something are also scaled (though likely not proportionally - again, writing good laws is hard). The think a fair consideration would be rate of replacement.
For fossils, it's really slow, so you can't take many without seriously depleting the resource.
For rocks, it's also very slow, but with a considerably large pool of resources. Some rocks are more important for erosion control than others.
For trees, it's a fairly fast regeneration and when don't strategically it can be sustainable.
For animals, some replace more quickly than others. Killing a mouse at a shelter is more sustainable than killing a bear due to their replacement rates.
For water, it replaces very quickly. I would guess that most people probably don't think of this as taking something, but it is. Of course, most of us return it within a few hours one way or another anyway.
So, there is a lot of gray, but that doesn't mean we can't discriminate between the shades. In psychology they call it the
just-noticeable difference (JND). Here, the colors are metaphorical, but the analogy is good enough (I hope). So, while taking fossils is likely unsustainable, taking a drink is (and the difference between them is pretty easy to see). Taking a fossil vs. taking a rock? The difference is less obvious I think. The
slippery slope fallacy often involves invoking the small differences between each step on the scale (e.g. fossils to rocks) to argue that there isn't a big difference between items further apart on the scale (e.g. fossils to water).
It's probably also worth stating that I'm don't disagree with most people here - I'm attempting to parse out broad, high-level assumptions (as are others, which is much appreciated).