It is not a fallacy, though it is also not as clear cut as...

It is not a fallacy, though it is also not as clear cut as rounded (alluvial) v sharp edges (pipe).
If you want to see smooth edged rough diamonds have a look at Ellendale goods.
You will find sharp edged diamonds right next to rounded or etched stones whether they are alluvial or from the source, you can really only judge it when looking at a larger parcel of diamonds and the % of stones affected.
The distance travelled, turbulence, how much diamond on diamond impact can all affect the diamonds.
I'm sure a diamond Geo will be able to give a more detailed analysis.