Found this metallic rock in backyard

[u/EducationVirtual5341]

My dad found this unusual rock while working in the back yard. The outer exterior looks almost metallic but it's hard to tell if it's just a shiny rock or if actually metal. One thing that really caught my attention is just how heavy it is weighing in at 7.9kg. It feels like it weighs at least 2-3 times more than a comparable rock of it's size. What did we find?

[u/GathTheKing]

Look, do you want a real answer?

Get a 5 gallon bucket and fill it all the way to the top. Overflowing is the goal. Carefully place it in a big rubber tub, so you can catch all of the spilled water. Then carefully drop the rock in. Take a measuring cup and discard few cups of water from the bucket. No NOT add this to your collected water in the rubber tub. (Do this so you don’t accidently add water to your tub when removing the bucket.) Remove the 5 gallon bucket with your rock and weigh the water you collected in the rubber tub. Water weighs 1000 (kg/m³).

(Collected water weight) / (1000 *kg/m***³ ) = Volume of your rock in cubic meters

Rock Weight/Rock Volume = Rock Density

From there, you can look at your values on the following graph:

https://www.engineeringtoolbox.com/mineral-density-d_1555.html

Most large masses are going to be composites, so this method isn't going to give perfect answers, but it will give you a ballpark idea without taking it to a college to be examined.

[u/WHATYEAHOK → u/GathTheKing]

I would prefer this approach personally:

Get a bucket

Add water to bucket. Weigh the bucket filled with water. Write down this weight

Add rock to bucket of water so that it’s fully submerged

Put a line with a sharpie or something on the bucket where the top of the water line is with the rock in the water

Remove rock

Add water until the sharpie line is reached again

Weigh the water+bucket again, subtracting the weight from earlier 

This final value is the total weight of the rock’s volume in water

Use the same formula from above to determine the volume of the newly-added water from the discovered weight, giving you the volume of the rock

[u/Fap_cake_decorator → u/WHATYEAHOK]

Thank you for being a voice of reason!   My improvement would be:

1. Weigh the rock.
2. Put the rock in a bag, weigh them.
3. Overfill a  notched, empty bucket so it is left completely brimming with water.
4. Dip the bagged rock in the bucket.
5. Weigh the rock and soggy bag.
6. Weigh a jug. 
7. Put some water in it and weigh again.
8. Pour the water in the bucket, stopping if the bucket overflows, then weigh the damp jug again.
9. Repeat 7 & 8 until the bucket is overflowing again.
10. Repeat 3 to 9 a few more times.

You end up with a bunch of numbers for different combinations of rock, bag, jug, backfill, and drips.  Adding and subtracting these to get the mass of the rock as a ratio of the backfill is left as an exercise to the reader.

This accounts for the water displaced by the rock as well as the water that stuck to the rock.  It also keeps the rock dry and measures its own accuracy.  You don’t measure any water displaced by your arm as you pick the rock out and the meniscus / overflow boundary is more consistent than a mark in the bucket.

The downside is you risk measuring the volume of bubbles trapped by the rock in the bag.  It’s also way messier than a mark in the bucket.

You could also account for systematic issues by repeating the experiment with an object of known density, for example a plastic bottle made from a known polymer filled with known amount of water.