Although this is not readily apparent, many substances that surround us every day are comprised of crystal clusters or assemblies. In some cases, nature illustrates this fact more conspicuously by allowing free-standing crystals to develop around hollow spaces. Much like trees on a mountain side, these crystals are firmly attached to the surrounding rock mass.
An even scarcer and eagerly sought-after rarity are double-ended crystals that manifest as a consequence of direct gas deposition. They develop without being attached to any kind of matrix, although the mechanics of this process are still not well-understood. Sometimes, they contain hollow regions that take on the form of a crystal; in extraordinary cases, these may turn out to be negative impressions of calcite crystals. They are created when calcium carbonate - calcite - crystals are initially embedded during the growth of the quartz crystal. At a later point, the calcite is dissolved by acidic solutions, and the quartz crystal continues its growth around the hollow space left behind, sealing in the surrounding substances such as tar, oil and natural gas. These so called three-phase inclusions remain preserved for millions of years for us to see. The gas bubbles trapped inside viridescent oil make for a curious natural bubble level.
Over the years, I have collected these wonderful specimens in anticipation of using them for a cross, which is now my next project.