Bismuth crystal. Bottom front edge has a length of about 3.5cm. Iridescent colour is created due to interference effects in a thin oxide layer, which forms when the hot crystal is pulled out of the bismuth melt.
<a href="//commons.wikimedia.org/wiki/User:Dschwen" title="User:Dschwen">Dschwen</a>
CC BY 2.5
<span class="int-own-work" lang="en">Own work</span>