I have a rock that looks like granite. It displaces 2 cups of water, weighs 6.2 lbs. and is round. What is it?

Question

Granite weighs 2.7 pounds for 2 cups and gold weighs 20.1 pounds for 2 cups. If it is gold and granite, it would be 80% granite and 20% gold. It was found in a rock quarry in British Columbia, Canada about 3 miles from an old gold mine. I want to know what is inside, for certain, without openning the rock.
Thank you for the help.

Answer 1

The first thing you need to do is calculate the rock's specific gravity based on the volume and mass that you have provided. You said it displaces 2 cups of water and weighs 6.2 lbs. Assuming the rock is not porous and does not have any unusual surface tension, that's quite simple. Given that all scientific measurements are in metric, you need to convert to metric: 2 cups is 472 cc and 6.2 lbs is 2.8 kg, or 2.8 kg per 472 cc. Divide the whole thing into one and you end up with a specific gravity of 6 (or a density of 6 g/cc).

There are not many rocks that are that heavy, and its definitely not granite - unless you've made a mistake in one or both of your measurements. Its more likely a mineral such as arsenopyrite, scheelite, or magnetite. Knowing the SG alone is not particularly useful for determining rocks and minerals, and 3 miles from an old gold mine doesn't mean a thing.

It will not be possible to be certain what's inside, without breaking it or drilling a hole in it.

If you are still determined to find out what kind of a rock (or mineral) you've got, try the geology department at your local university.

Answer 2

I don't get what you mean by displaces water, I've never heard of that test before. I think it's supposed to mean that 2 cups of granite weighs 2.7 lbs and 2 cups of gold would be 20.1 lbs. But that doesn't sound right either. I'm an Earth Science major too, and I've never heard of this. They only way you'd be able to tell what is in your rock is to crack it open. Sorry.

It could be gold, you'd have to ask someone who can look at it. Also, it would not be able to grow inside the granite, it would only form on the outside. Because granite is an igneous rock, it would have solidifyed completely before the gold has even had a change to form on it.

Gold usually only grows in quartz veins and is found in alluvial deposits. Granite does have quartz in it, so it might be likely. But it would have to be a pretty big piece. Most likely not. Gold is also found near pyrite, so it may be that. If it's gold it would be yellow, like a mustard yellow. This is what true gold looks like. Pyrite (fools gold) is a brassy, darker yellow.

Answer 3

When a rock is totally under water, all it displaces is it's volume. Density and weight have nothing to do with it. A two liter jug filled with gold will displace the same amount of water as a two liter jug filled with granite when they are both under water =2 liters.

Only if they are located in a floating object, can you try to determine their densities by the amount of water they displace

Answer 4

Your rock could be absolutely anything that displaces 500ml and weighs 2.7 pounds (1.2 kg). It could also have just about any mineralogy. Besides granite does not usually carry gold and the density of your rock does not indicate anything unusual metallurgicaly speaking.

Answer 5

I'd think that if your rock did in fact contain gold, then some of it should show on it's surface.

However, since granite in its natural state doesn't contain any pure metals, you might try running a metal detector over it to see if anything registers.

Since you mentioned that it's round, it's possible that you've got a geode.

Answer 6

Compute its density and check the chart.