OK, well how heavy is it?
...and by that I mean what is the rock's specific gravity or density?
(weight verses water volume displacement)
The SG of rock can be measured using this method:If you have a reliable accurate scale you can do this yourself.(1/10oz or 1g incr. or resolution =/<1% of the rock's total weight)
- Weigh the rock dry and then record the result. _______
- Select any suitable container that you can suspend the rock in so it can be just completely submerged in (cold) water without touching the bottom or sides. Try to shoot for a container that is large enough to accomodate the rock without causing water to spill over when you submerge it.
* It is imperative that the rock not get wet in any way before doing this if you want an accurate result, just eyeball estimate the size and displacement of the container needed.
- Fill the container to a level where the rock can be completely submerged without causing the forementioned spillage or touching the bottom or sides.
* It is really important that no water is "lost" during the procedure too or it will throw off your results.
- Weigh the container filled with water and record the result or tare out the container of water on the scale. _______
* This is where it can get a bit tricky.
- Tie some fishing line around the rock so you can dangle it in the container of water while it's still on the scale, you need a fairly steady hand for this or the scale read-out may get jumpy on you. Do it slowly and carefully to avoid any spillage.
- Record the weight increase once the rock is completely submerged but not touching the bottom or sides. The weight increase should be noticeably lower than the weight of the dry rock in air. _______
- Subract the "dangling under water weight" from the "dry weight" and this will give you the "water bouyancy differential". _______
- Divide the dry weight by the bouyancy differential weight and this will give you a fairly accurate measure of the rock's specific gravity.
* It is also important to make sure that any voids in the rock have a chance to "fill in" with water completely and that you don't have air bubbles clinging to the rock surface when you weigh it under water or the bouyancy measurement will be thrown off.
Measurement examples:
If the rock weighs
250g dry and causes the weight of the container to increase by
150g when you dangle it under the water, then the bouyancy differential is
250g-
150g=
100g.
250/
100=
2.5, your rock's specific gravity in this case. This indicates the rock has a density of 2.5g/cm^3.
If the rock weighs
250g dry and causes a weight increase of
200g when you dangle it under the water, then the bouyancy differential is
250g-
200g=
50g.
250/
50=
5, your rock's specific gravity in this case. This indicates the rock has a density of 5g/cm^3.
A neutrally bouyant object (one that neither floats or sinks) has a specific gravity of '1' since it displaces the same amount of water as it's weight. (1ml of water weighs 1g)
The SG of an object that floats is less than 1 and can not be measured directly in this way (unless i'm mistaken).
Basic Volcanic rock is usually less dense than other stone types so this may help to narrow it down for you. If it has an ashy or slightly sulfery smell when you wet or cut it then it's probably a basic volcanic stone. If it is too dense to be basic volcanic it could be partially metamorphosed volcanic or some other type, maybe sedimentary by the looks of it.
It certainly does have the outward appearance of pumice though, just looking at the pic, but if you really want to know what it is you may need to do some more tests on it.
Rob