class BigDecimal

BigDecimal.json_create(z) # => 0.2e1
BigDecimal.json_create(y) # => 0.2e1
BigDecimal.json_create(x) # => 0.2e1

\Method +JSON.create+ deserializes such a hash, returning a \BigDecimal object:

z = BigDecimal(Complex(2, 0)).as_json # => {"json_class"=>"BigDecimal", "b"=>"27:0.2e1"}
y = BigDecimal(2.0, 4).as_json # => {"json_class"=>"BigDecimal", "b"=>"36:0.2e1"}
x = BigDecimal(2).as_json # => {"json_class"=>"BigDecimal", "b"=>"27:0.2e1"}
require 'json/add/bigdecimal'

returning a 2-element hash representing +self+:
\Method BigDecimal#as_json serializes +self+,

see Marshal[https://docs.ruby-lang.org/en/master/Marshal.html].
to serialize and deserialize a \BigDecimal object;
Methods BigDecimal#as_json and +BigDecimal.json_create+ may be used

def as_json(*)
  {
    JSON.create_id => self.class.name,
    'b'            => _dump.force_encoding(Encoding::UTF_8),
  }
end