minecraft-assets/data/minecraft/worldgen/configured_feature/poison.json

{
  "type": "minecraft:delta_feature",
  "config": {
    "contents": {
      "Name": "minecraft:light",
      "Properties": {
        "level": "15",
        "waterlogged": "true"
      }
    },
    "rim": {
      "Name": "minecraft:slime_block"
    },
    "rim_size": {
      "type": "minecraft:uniform",
      "value": {
        "max_inclusive": 2,
        "min_inclusive": 0
      }
    },
    "size": {
      "type": "minecraft:uniform",
      "value": {
        "max_inclusive": 7,
        "min_inclusive": 3
      }
    }
  }
}