Oval Border

import 'package:flutter/material.dart';

const SizedBox(
  height: 80,
  width: 60,
  child: Material(
    color: Colors.amber,
    shape: OvalBorder(
      eccentricity: 1,
      side: BorderSide(
        color: Colors.purple,
        width: 2,
      ),
    ),
  ),
),