Geo 730: September 8, Day 616: Milky Quartz

When not contaminated by impurities, quartz tends to be colorless and clear, or milky-white. I can't say with any authority what differences in environments lead to one or the other, but my guess is that transparent quartz is likely the result of slow, orderly, relatively low-temperature deposition from aqueous solution, while milky quartz results from rapid deposition, with resulting fluid inclusions (Too small to see here, but I have seen macroscopic examples elsewhere, in the related Blue River District- PDF, see page 5 for location.) and flaws in the crystal lattice. These inclusions and flaws would lead to light scattering in the crystal, so they're translucent: light passes through, but is disrupted enough that you can't see an image through it.

Photo unaltered. August 5, 2012. FlashEarth Location.