Continuous Rectangle Border

import 'package:flutter/material.dart';

SizedBox(
  height: 60,
  width: 60,
  child: Material(
    color: Colors.amber,
    shape: ContinuousRectangleBorder(
      borderRadius: BorderRadius.circular(30),
      side: const BorderSide(
        color: Colors.purple,
        width: 2,
      ),
    ),
  ),
),