How to Choose The Best Beach Pebbles for Polishing
Many people have collections of pretty beach pebbles saved from vacations.
Others live near our coastal areas and regularly walk along the shoreline.
With the advent of the affordable home stone tumbling machines, we can now polish our pretty beach pebbles for posterity.
It is, however, of vital importance to know which beach pebbles can take that wonderful shine we want to give them. Some stones will simply not polish up, no matter how much we try.
- Too soft and they will break up.
- Too brittle and they will break up.
- Too many cracks and they will not polish well.
We do not need to have the knowledge of a geologist to know what type of stones these pebbles are. All we need to know is which ones are best for polishing.
With the aid of the internet, we can at least partly identify the rock types by color, hardness and then by specific gravity.
Tips for choosing Pebbles on the Beach for Polishing
- Look at the wet stones below the tide line. Dry stones tend to lose their color. Take care to keep an eye out for big breaking waves that can reach where you are walking and drag you out to sea. Roughly every seventh wave will come right up to where you are on the wet stones.
- Pick stones that you like the color of.
- Choose really smooth stones. If 50 million years of wave and tidal action didn't smooth that stone off, you will have no chance of doing so in a rock tumbler for a week. Rough stones are a sign of softness.
- Buy a book, check your local library or look online for information relating to the types of semi-precious stones available in your area. Semi-precious stones tumble well. They also have a re-sale value.
I have the book shown here - Collecting Rocks and Minerals by Patti Polk and it is an extremely useful book for people wishing to to learn more about rock identification, values and their lapidary uses. If you are interested in polishing beach pebbles, I strongly suggest you buy it.
How to test translucency of stones
How to Identify stone types
The first thing I would suggest you do is buy a good book on identifying rocks and minerals.
While I have several, I find that in the main they show you the polished versions of the rocks in questions, or photos of them in their raw state as they are mined from some underground cave, or as found on the surface in some remote spot.
Unfortunately, this is not much help when you have found a small rounded and smooth fragment on a beach, that looks nothing like those photos.
Your book should tell you the chemical composition and specific gravity of the stone in question, which could help you in your final analysis.
Thngs that will help you identify your beach pebble
- Color - is it a pure color, a mixed color, or just a dull color? The prettiest tumbled stones are usually colored and not just a dull gray.
- Texture - is it smooth all over, or does it have cracks and fissures? Choose stones which are as smooth as possible for polishing.
- Weight - is it heavier or lighter than other similar stones?
- Opacity - can you see through it, or is it solid? Many crystalline gemstones are translucent and reflect the light.
- Specific gravity - read on to learn how to check the specific gravity of a stone. This is only required when you have narrowed down the possibilities of the type of stone you have, and the ones left have different readings.
- Hardness - check below on how to test hardness, which is measured on the Mohs scale. This is vital to know when polishing groups of pebbles together.
Archimedes and his Eureka moment
Specific gravity and the Archimedes Principle
Back around 250 B.C., famous Greek mathematician Archimedes had been troubled by a task set to him by King Heiro II of Syraceuse.
The king suspected that the goldsmith who crafted his crown had stolen some gold and substituted it with silver.
The crown was so intricate it was impossible to tell.
One day Archimedes was getting in the bath, and noticed the water displacement caused some of it to spill over the top, his genius mind suddenly realised the answer to his problem.
By collecting up the water and measuring it, he was able to calculate that the space in the bath taken up by his body was equal to the volume of water spilt.
So excited was he by this discovery, he reportedly ran naked (straight from his bath) through the streets of Athens shouting "Eureka! Eureka!"
Next time you have a eureka moment, you will think of this!
Archimedes then applied his principle to the crown.
He weighed it out and then weighed out an equal block of pure gold.
He first placed the gold block in a volume of water and noted where the water rose to.
Then he placed the crown in an identical volume of water, and watched as the water cascaded over the sides.
Gold is denser and heavier than silver, so it would take up less room in water.
The King was right. He had been cheated.
Graduated cylinder at Amazon
How to measure the specific gravity of a stone
Using the exact same principle as Archimedes, firstly weigh the stone.
Then place it in a volume of cold water, the optimum temperature for accuracy being 4ºC (water from the refrigerator should be around this mark).
It is best to use a graduated cylinder which will have small amounts marked off in millilitres.
Note the rise in water level when the stone is put in.
If, for example, you put in the stone into 20mls of water, and it rises to 22.2, then you subtract the two numbers, leaving you with 2.2.
Say the stone weighed 6.48 g.
The sum is then 6.48 / 2.2 * 1000 = 2945.45 g/L
You then divide the sum by 1000 leaving you with a specific gravity of 2.945.
Most stones have a specific gravity of less than 3.
Hardness Rating of Stones
Rocks suitable for tumbling
Rocks suitable for tumbling fall in the mid-range of the above scale.
Any harder than 7.5 or softer than 5.5 will simply not tumble well.
Fortunately, most beach pebbles that we know will shine up beautifully in a stone tumbler are within this range.
Hardness Rating of some common tumblestones
Obsidian, Turquoise, Glass
Opal, Moonstone, Sodalite, Hematite
Spodumene, Garnet, Jade
Quartz, Amethyst, Citrine, Agate, Aventurine
Beryl, Emerald, Aquamarine, Zircon
Scoring a stone with materials of different hardness levels
How to tell how hard the stone is
The reason why it is so important to know the hardness rating of stones is because you want to tumble similar groups together.
Hard stones will need tumbled for longer, and softer stones need a shorter tumbling time.
A simple test to help you find out the hardness of the stone you have, is to score it with your penknife.
The steel blade has a hardness level of between 5.1 and 5.5 on the Mohs scale.
If your stone is harder than that, it will not leave a scratch.
Look at the photo here where I have scored a lump of sandstone with both a penny (3 on the Mohs scale), and a knife blade.
Both scratches show, which tells me that sandstone is softer than 3, and will therefore not tumble well.
Quartz has a hardness rating of 7, so if you can get a hold of some sharp quartz (it is the most common mineral on the planet; there is probably some on waste ground near you), you should be able to use it to scratch most beach stones that are not scratched with your penknife.
- a fingernail has a rating of 2
- a penny has a hardness rating of 3, and will easily scratch soft rocks.
- a steel blade has a rating of 5.5, and will scratch rocks softer than that.
- a steel nail file has a rating of 6.5
- Quartz has a rating of 7 and will scratch rocks less than 7.
Jet stones and how I identified them
There are a number of deserted pebble beaches within a 10-15 minute drive from my house, and I have collected a huge variety of stones for tumbling.
Among them are many shiny, glassy black stones which when I tried to identify them, was able to narrow it down to just three rock types.
One was obsidian, one was tourmaline, and the other was jet (Molvalite).
- Tourmaline has a Mohs rating of 7 - 7.5
- Obsidian has a Mohs rating of 5.5
- Jet has a Mohs hardness rating of 2.5 - 4.00
A scratch test isn't terribly helpful. The scratch barely shows, because underneath the surface is pure black too - jet black in fact.
One thing very noticeable in my stone is that is stays completely black even when the surface is dried. All my other black stones turn grayish.
I held a lighter underneath it, and after a minute or so I could smell tar.
As a final test, I rubbed the stone vigorously and then held it near torn up paper.
The paper rose to the stone.
Jet becomes attractive to objects when rubbed hard, preferably not with your fingers because it gives off black or brown markings.
Geologists carry a small white tile around, on which they can try and 'draw' with their rock samples. Some rocks leave markings.
Out of the known semi-precious stones, there are also jaspers and agates dotted about for the eagle-eyed spotter.
Jet polishes very well if it gives off black markings. Ones with brown markings are softer and harder to polish.
Jet isn't really a stone. It is made from fossilized wood dating from 180 million years ago.
The art of polishing beach stones video
Have you tried polishing beach stones?
All photographs on this page belong to me unless otherwise stated.