Diamant cu CSS

<style type="text/css">
#diamond {
 width: 0;
 height: 0;
 border: 50px solid transparent;
 border-bottom: 20px solid #05ed08;
 position: relative;
 top: -50px;
}
#diamond:after {
 content: "";
 position: absolute;
 left: -50px;
 top: 20px;
 width: 0;
 height: 0;
 border: 50px solid transparent;
 border-top: 70px solid #05ed08;
}
</style>

<div id="diamond"></div>

Diamant 2

<style type="text/css">
#diamond {
 border-style: solid;
 border-color: transparent transparent #0809fe transparent;
 border-width: 0 25px 25px 25px;
 height: 0;
 width: 50px;
 position: relative;
 margin: 10px 0;
}
#diamond:after {
 content: "";
 position: absolute;
 top: 25px;
 left: -25px;
 width: 0;
 height: 0;
 border-style: solid;
 border-color: #0809fe transparent transparent transparent;
 border-width: 70px 50px 0 50px;
}
</style>

<div id="diamond"></div>