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Had a discussion with somebody about what a single piece of gravel is?

Would it be a rock fragment, pebble, grain of gravel/rock? Or is gravel both the singular and plural (like pants for trousers)?

A piece of gravel would indeed be a 'piece of gravel'.

The reason for this is that your other suggestions rule themselves out for various reasons, the primary of these being that not all gravels are alike.

'Grain' would not apply to pieces of stone larger than a couple of millimeters at most, 'pebble' would not apply to a piece of crushed rock and a piece of crushed rock would not be called a pebble.

So while all those terms come under the wider set of 'gravel', they are not interchangeable.

Gravels come in two principal forms; 'sharp' and 'pea' or 'pebble' gravel.

'Sharp gravel' is crushed stone, ie the angular broken pieces you referred to as 'rock fragments'. Sharp gravels are quarried from monolithic rock beds. Mostly from igneous or metamorphic rocks, though there are sedimentary rocks which produce useful gravels, but you wouldn't use the softer sandstones.

The crushed rock is generally screened to a tight range of sizes so that all of the pieces are much of a muchness, but it also covers what variously gets called 'scalpings', quarry bottoming', 'quarry process', 'crusher run' etc which is unscreened gravel and will include all the sizes below the stated maximum and be described by its maximum size and 'to dust'. So an unscreened, unwashed gravel might be specified as '50mm to dust'. You might call the dust 'grains' but you wouldn't apply the term to a larger piece.

A surface laid with a single size sharp gravel will remain a relatively loose and free draining surface. A gravel which goes down 'to dust' can be laid to a more compacted surface which will still drain, but if there are dips puddles can form. Once you have puddles forming the smaller 'dust' will tend to migrate to the surface and create a seal, reducing the free draining properties.

Pea and pebble gravels are quarried from deposits of stone which has been rounded by natural processes, ie by water in river, glacial or wave situations. The rounded shape gives it different characteristics in use and it will not lock under loading as sharp gravels will - imagine the difference between ball bearings and pyramid shaped blocks if you were trying to roll something flat across them. There are the gravels whose individual pieces you would call 'pebbles'.

So, a piece of sharp gravel is a fragment of broken rock and a piece of pea gravel, shingle or pebble gravel is a pebble, or a piece of shingle.

There is no overarching term with which to describe a piece of gravel, which encompasses sharp, pea, pebble and crusher run other than 'piece of gravel' or perhaps 'piece of aggregate' if you are more construction-industry minded.

I'm sure the Wikipedia entry covers the same ground.

The only real alternative to "piece of gravel" and even then only in some situations, is "stone".

If (for example) it's stuck in your shoe scratching the floor, or if you've stepped on one and hurt your foot, you don't really care that it came from a mass of gravel. This covers both pea and sharp gravel, but isn't appropriate if you're using it in construction or gardening

Its neither singular nor plural, like sand.

For sand, you'd say a grain. For gravel, probably piece or (a bit slangy) bit.

"My shoes scratched the floor because there was a piece of gravel in the treads."

The word choice is going to depend on the type of gravel, and probably the proximity of the "piece" to a larger pile of gravel.

Some gravel is made of whole, small stones and some is made of crushed rock of various types (granite, limestone, etc). If there is a large pile or pathway or planting bed of gravel within sight and you find a small piece that obviously came from the pile, you would most likely say "Look, I found a piece of gravel".

If you're walking around and find a small rock, and there's not an obvious pile of pebble gravel nearby, you're going to say, "Look, I found a pebble." In this case saying that you found a piece of gravel is going to be too specific. That pebble could have come from anywhere and there's nothing that really identifies it as a piece of a larger whole.

I used pebble in that example, but isolated pieces could be called "a piece of crushed stone" or "a rock" or similar depending on the piece you found. Also, using the specific word rather than "piece of gravel" is correct even if there is a pile of gravel nearby.

So, to sum it up, if there is gravel nearby, and you're not sure what type of rock it is, you can certainly say "a piece of gravel". If you know what to call the specific rock or fragment, or it's found separately from a pile of gravel, use something like "a pebble" or "piece of crushed stone". Calling it "a grain" or "a gravel" is not correct.