"blue metal" meaning in English

{
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      "expansion": "blue metal (uncountable)",
      "name": "en-noun"
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Australian English",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
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        {
          "kind": "other",
          "name": "New Zealand English",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc; gravel; road metal."
      ],
      "id": "en-blue_metal-en-noun-MwVw7dMa",
      "links": [
        [
          "gravel",
          "gravel"
        ],
        [
          "road metal",
          "road metal"
        ]
      ],
      "raw_glosses": [
        "(Australia, New Zealand) Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc; gravel; road metal."
      ],
      "tags": [
        "Australia",
        "New-Zealand",
        "uncountable"
      ]
    }
  ],
  "word": "blue metal"
}

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "blue metal (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "Australian English",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns",
        "New Zealand English"
      ],
      "glosses": [
        "Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc; gravel; road metal."
      ],
      "links": [
        [
          "gravel",
          "gravel"
        ],
        [
          "road metal",
          "road metal"
        ]
      ],
      "raw_glosses": [
        "(Australia, New Zealand) Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc; gravel; road metal."
      ],
      "tags": [
        "Australia",
        "New-Zealand",
        "uncountable"
      ]
    }
  ],
  "word": "blue metal"
}