Block Shapes

Block shapes are essentially block geometries that are hard-coded into vanilla, meaning that they exist without having accessible files. The geometries themselves cannot be overriden, nor new ones created - but existing ones can be assigned to most vanilla blocks through a resource pack.

Application ​

They are assigned using blocks.json, under child "blockshape" of a block's object as a string.

"minecraft:dirt": {
    "blockshape": "block",
    "sound": "gravel",
    "textures": "flattened_dirt"
}

List of known Blockshapes ​

Original Credit