div{
  width:100px;
  height:100px;
  background:tomato;
  position:relative;
  top:100px;
  left:200px;
  transform:rotate(45deg);
    
}
div:before{
  content:"";
  width:100px;
  height:100px;
  background:tomato;
  border-radius:50%;
  position:absolute;
  top:-50px;
  left:0px;
  
}