Which of the following describes a solid crystal inclusion?

A needle inclusion refers to a type of solid crystal inclusion that is typically elongated and slender, resembling a needle. These inclusions form during the crystallization process and can be composed of minerals such as rutile, tourmaline, or other substances. Needle inclusions are important for gemologists as they can help to identify the type of gemstone and offer insights into its formation and growth history. Their shape and orientation can affect the appearance and optical properties of the gemstone, making them significant in the study of colored stones.

The other options do not fit the definition of solid crystal inclusions. A mottled gemstone relates to the surface color or texture rather than internal structured inclusions. A negative crystal refers to a void or cavity that takes the shape of a crystal but lacks any solid material. A natural pearl is an organic gem formed by mollusks and does not exhibit solid crystal inclusions as found in minerals. Each of these options illustrates different aspects of gemstones or inclusions, but only the needle inclusion specifically characterizes a solid crystal inclusion.