Hollandite star in quartz crystal. Hollandite is a rare mineral of oxides class. Occasionally it forms specific in form inclusion in quartz crystals, like this. If you look closely, you can see that the hollandite star is located on the face of the phantom quartz crystal. Width of the star is 2.4 mm, the face through which the photo was taken is polished.
<a href="//commons.wikimedia.org/wiki/User:Boris_Lobastov" title="User:Boris Lobastov">Boris Lobastov</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>