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can anybody tell me what kind of stone this is ?

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Agate is quite a bit harder than glass. Tougher too. So you can do some scratching with a piece of glass and come to some fairly good conclusions.

It is either melted glass or agate. Both are fairly common. A lot of wine bottles get melted on the beach and make pieces that look just like that. But many agates look just like that too.


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A melted wine bottle will weigh a bit less than an agate of the same mass. 

Your specimen is made of silicon doixide. Both glass and agate are basically the same stuff. So there is not much difference in weight and many physical characteristics will be similar. 

The weight of your specimen is irrelevant. We don't know the mass.  The specific gravity is important and That is weight compared to mass. That is a fairly simple thing to determine but you need a scale and a method to weigh the object and then weigh it suspended in water. From there it is a simple calculation.

Whatever the specific gravity is your specimen is vitrified silica. "Glass" for all practical purposes. Either man made or natural. If it wieghs 3.3kg. It is probably a chunk of man made glass. Agates that big are fairly rare without lots of inclusion and cortex.

Do a specific gravity calculation. Check out the expected specific gravity of agate. Then compare it with glass. Look at the moh's hardness scale and look at the difference between glass and agate. 

Agates don't have bubbles in the matrix. Glass often does. So little bubbles indicate glass. Glass is more brittle and chips easily. Some pressure with a pointed tool on similar edges will quickly distinguish glass from agate.

There are lots of chunks of glass in the ocean. Ships are floating foundries. And tons of glass has been melted on the beaches. So IMHO it could be a chunk of glass.

So there is my opinion on your 7 lb. Chunk of vitrified, microcrystalline silica. Whatever it is it is that. So I hope that gets you a little closer to an identification.

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Then it is probably an agate. 

Glass, even volcanic glass (obsidian) is softer.

It is not quartz because it is not crystalline. It may be cryptocrystalline like agate or amorphous like obsidian. Glass is also amorphous.

It could be a smoky agate but after you told us it weighs 3.3kg. I would doubt that is an agate. Most agates have banding and inclusions. An agate that big would not be that uniform and translucent.

You must observe the basic physical characteristics to identify what it is. We can't do that from a photo. The best any of us could do is tell you it is silicon dioxide, amorphous or cryptocrystalline. I can make a wild guess based on the "cast looking" surface it is glass. I can assume the inclusions are bubbles indicating man made glass. And the size indicates a big chunk of glass too. 

Only you can determine hardness and specific gravity. And that is the only way you are going to know exactly what it is.

If you can't scratch it then use it to scratch something else of a known hardness. Learn how to use the scratch test to determine hardness. If you think it is heavy for its size do the specific gravity. With an accurate estimation of hardness and density this would be easy to identify. Without that all we can tell you is that it is made of silicon dioxide.


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