Estimate as water and fudge it a bit. Conveniently the fudge factor is just the specific gravity and is already tabulated as such in a lot of fields.

But it turns out that water is a pretty good bet most of the time:

- Settled snow is around 0.25

- Dried wood is around 0.5

- Soil is around 1.2

- Rock is around 2.5

Which is pretty good if you want to answer "how much does that truck / ship / mountain / lake weigh?".

Of course there are some anomalies: Tungsten is around 20, but it's not like imperial units help here, and the name literally translates to "heavy rock".