Result

ダイヤモンドの形のボタン。
シンプルに、オブジェクトを回転させ、テキストだけ元の角度に戻してます

css

.container .square {/*基本設定。回転させておく*/
  width: 100px;
  height: 100px;
  position: absolute;
  color: white;
  border: 2px solid white;
  transform: rotate(45deg);
  display: table;
  cursor: pointer;
  transition: ease 0.5s all;
}
.container .square:hover {
  background: white;
  color: black;
}
.container .square p {/*コンテナ内のテキスト。回転させた親要素と同じ数値の角度を戻す*/
  display: table-cell;
  vertical-align: middle;
  text-align: center;
  transform: rotate(-45deg);
}
.container .square:nth-child(1) {/*各位置調整*/
  top: 0;
  left: 0;
}
.container .square:nth-child(2) {
  bottom: 0;
  left: 0;
}
.container .square:nth-child(3) {
  right: 0;
  top: 0;
}
.container .square:nth-child(4) {
  right: 0;
  bottom: 0;
}
.container .square:nth-child(5) {
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%) rotate(45deg);
}

html

<div class='container'>
  <div class='square'>
    <p>ONE</p>
  </div>
  <div class='square'>
    <p>TWO</p>
  </div>
  <div class='square'>
    <p>THREE</p>
  </div>
  <div class='square'>
    <p>FOUR</p>
  </div>
  <div class='square'>
    <p>FIVE</p>
  </div>
</div>

via

DIAMONDS