A black stone with contrasting white spots or inclusions is common in geology, but its identity depends on its formation and mineral composition. This distinct, speckled pattern appears across multiple geological categories, including igneous rocks formed by rapid cooling and those formed by secondary mineral deposits. Correct identification requires examining the texture and composition of both the black matrix and the white inclusions.
Dalmatian Stone: The Primary Identification
The stone most frequently associated with the black-on-white description is Dalmatian Stone, often sold commercially as Dalmatian Jasper. This material is named for its speckled appearance, which resembles the coat of the Dalmatian dog breed. While historically misclassified as jasper, Dalmatian Stone is actually a type of igneous rock.
The light-colored matrix is predominantly a mixture of albite, a plagioclase feldspar, and smaller amounts of quartz. The distinct black spots are inclusions of arfvedsonite, an iron-rich amphibole mineral. These spots are typically irregular in shape and scattered throughout the rock mass. The material is relatively soft, registering a hardness of about six on the Mohs scale. Because of its unique aesthetic, Dalmatian Stone is widely used in jewelry, such as tumbled beads, cabochons, and decorative carvings.
Igneous Rocks with Speckled Patterns
Beyond the Dalmatian Stone, several rocks formed from cooling magma or lava also exhibit a black-and-white speckled appearance. These igneous rocks are geologically distinct, forming under different temperature and pressure conditions.
One notable example is Snowflake Obsidian, which is a volcanic glass formed by the rapid cooling of silica-rich lava. The black portion is the glassy matrix of the obsidian, which lacks a crystalline structure because of its fast formation. The white “snowflakes” are clusters of needle-shaped crystals called cristobalite, a high-temperature variety of quartz. These spherulites form as the volcanic glass begins to devitrify, or transform into crystalline material over time.
Intrusive rocks, which cool slowly deep within the Earth, also fit this pattern, such as Diorite and its fine-grained equivalent, Diabase. Diorite is a coarse-grained rock often described as having a “salt and pepper” look because of its large, visible crystals. The black minerals are typically hornblende or biotite, while the white crystals are plagioclase feldspar. Gabbro is a similar, darker rock where the black matrix of pyroxene contains white crystals of calcic plagioclase feldspar.
Practical Ways to Distinguish the Stones
Distinguishing between these black-and-white stones requires examining their fundamental physical properties, especially texture and luster. Snowflake Obsidian has a characteristic vitreous, or glassy, luster and often exhibits very sharp edges if recently fractured. It is also the least hard of the three main types, with a Mohs rating of 5 to 5.5, and feels non-crystalline to the touch.
Dalmatian Stone, despite its igneous classification, tends to have a more earthy or microcrystalline texture due to its fine-grained matrix of feldspar and quartz. The black inclusions appear as irregular, rounded spots rather than angular crystals. If the stone is an architectural material, like a countertop or floor tile, it is more likely to be a type of Gabbro or Diorite. These intrusive rocks are very hard and dense, and their white spots are angular, interlocking mineral crystals that reflect light distinctly.