A few months ago a story did the rounds about a “negative crystal floating in space”  which was accompanied by a rather spectacular image (Image 1).
Image 1: Negative crystal in spinel. Image by Danny Sanchez
This picture was described as a negative spinel and was one of the most perfectly formed crystal inclusions found and photographed by Danny Sanchez using photomicrography techniques . This unique photography technique allows small regions within crystals to be photographed with exceptional clarity, creating extraordinarily surreal images of crystal inclusions. Negative crystals aren’t the only type of crystal inclusion however, and they certainly aren’t the only ones to produce stunning images.
Generally speaking, negative crystals are a specific kind of crystal inclusion. Inclusions are when a foreign material is trapped within a host crystal. The foreign substance can be solid, liquid or gaseous and can become trapped within the crystal either during its growth or once the host crystal has formed. The exact method of inclusion formation depends on the type of inclusion and the conditions under which the crystal was grown .
The most well known solid inclusions are ancient insects or plant life suspended in amber. However, solid inclusions can also include different types of gemstones embedded inside another crystal, typically quartz (Image 2).
Image 2: Rutile on hematite in quartz. Image by Danny Sanchez
These mineral inclusions either form simultaneously with the host crystal or were pre-existing and the host crystal has grown around it, encasing the foreign material. In the case of quartz which usually forms in hydrothermal conditions (basically a hot water soup of dissolved minerals) the included gem existed in the solution prior to the growth of the quartz and over time the quartz grows around it (Image 3).
Image 3: Purple and blue fluorite in quartz. Image by Danny Sanchez
Alternatively, as many crystal growth conditions are under high temperature and pressure conditions and in aqueous media, liquid inclusions highly saturated with dissolved minerals can form. As the crystal cools these minerals begin to precipitate forming a second crystal inside the host.
As mentioned, many famous crystals (such as the Naica crystal caverns) were formed in hot, mineral-rich water solutions. As a result, sometimes this solution can become trapped inside the crystal during growth. A rather unique liquid inclusion which occurs under pressurised conditions is liquid CO2, which remains trapped in liquid form due to the pressure maintained inside the inclusion.
Quite often liquid inclusions come paired with a bubble of gas, such as in Image 4 in which liquid petroleum and a bubble of methane are trapped in quartz.
Image 4: Petroleum and methane bubble in quartz. Image by Danny Sanchez
Finally, “negative crystals” form when a pocket of air is trapped within a crystal. These gaseous inclusions can be a specific gas, depending on the conditions the crystal grew in, or just plain old air. These inclusions can form either due to crystal growth which occurs in multiple directions that then intersect or because crystal growth in a particular direction is inhibited temporarily. Since gaseous inclusions are enclosed by crystal faces the shape will reflect the crystal habit (or defined external shape of the crystal) of the host and they are always oriented parallel to the host crystal (Image 5). While they may look like a typical gemstone, it is actually an inversion with air “inside” the crystal boundaries and crystal material on the “outside”. Hence these inclusions are named “negative crystals”.
Image 5: Negative crystals in amethyst. Image by Danny Sanchez
A special aspect of any type of inclusion is that the foreign material is suspended in time. In this way, the inclusion contains a “piece of the past” which provides insight into what the earth and the environment were like when the crystal grew, often millions of years ago. For example, the discovery from air inclusions in amber that the oxygen content in air reached 35% during the Cretaceous period, before suddenly dropping to near the current level of ~21% .
There are many other forms of inclusions in addition to those shown. Quite often they can produce stunning visual effects and increase the value of the gemstone. For example, star sapphires which produce a unique six-rayed star effect under certain light sources contain tiny inclusions of aligned needle shaped rutile (Image 6).
Ordinarily, most inclusions are microscopic in size and can only be properly visualised under a microscope. For the majority of images featured here highly specialised and expensive equipment is required.
More images of crystal inclusions can be found in Gübelin and Koivula’s Photoatlas of inclusions in gemstones  or at http://www.dannyjsanchez.com/.
 Hooper, R.; New Scientist 2014, 2975, 24.
 The Art of Photomicrography: Gemstone Inclusions by Danny Sanchez <http://www.dannyjsanchez.com/> (Accessed 11/14)
 Benz, K. W.; Neumann, W. Introduction to Crystal Growth and Characterization; Wiley, 2014
 Kump, L.R.; Kasting, J.F.; Robinson, J.M. Global and Planetary Change 1991, 5, 1.
 Gübelin, E.J.; Koivula, J.I. Photoatlas of Inclusions in Gemstones; Vol 1-3, Opinio Publishers