Many times semi precious gemstone agates and sometimes other gemstones are hollow, and sometimes there are "pockets" or cavities formed when the rocks formed. In such cases the center of the agate can consists of quartz crystals directed towards the free space forming a geode, or the pocket or cavity becomes crystal lined.
A layer of very fine crystals on a rock surface is known as "Druse" "Drusy" or "Druzy". The term Druse cabochon or Druzy stone is commonly used for any type of semi precious stone cabochon that hasn't been ground down, and has natural crystal or crystal quartz formations on top. Often the Drusy Cabochon will be the color of the mineral underneath. There are many beautiful variations: Drusy Chrysocolla is a beautiful bright blue, Pink Drusy are pink stones known as Cobalto Calcite or Cobaltocalcite, Green Drusy is Uvarouvite Garnet, Black Drusy is Hematite or dyed black Brazilian Agate, etc. and there are many man-enhanced varieties on the market too. These are usually natural agate druzy stones that are "coated" with a substance such as titanium, which makes a beautiful rainbow infused purple color.
Druse cabochons are truly "designer cabochons" as the material must be handcut to follow the crystals-so they are usually freeform shapes and make for very unique semiprecious stone cabochons.