Raw sapphire, in its unrefined state, is a variety of the mineral corundum, a crystalline form of aluminum oxide (\(\text{Al}_2\text{O}_3\)). Identification relies on a combination of visual inspection and physical testing, primarily due to its unique formation and composition. Recognizing genuine raw sapphire involves moving beyond simple color and shape, focusing instead on the scientific characteristics that distinguish it from other minerals. This guide utilizes the inherent properties of corundum to confirm its identity.
The Crystalline Structure and Shape
Raw sapphire crystals belong to the hexagonal crystal system, which dictates the specific geometric shape they naturally form. The most typical habit is a six-sided prism, often described as a barrel or spindle shape, which is wider in the middle and tapers toward the ends. These crystals often display distinct horizontal striations or growth lines across the faces of the prism, which is a strong visual indicator of corundum.
Even specimens collected from alluvial deposits, where they have been tumbled and worn, frequently retain a hint of this original hexagonal outline. The crystal terminations can be flat or pyramidal, but the overall morphology remains strongly suggestive of its corundum lineage. Identifying these remnant six-sided forms is a foundational step, helping to rule out amorphous or differently structured minerals immediately.
Observing Color, Luster, and Transparency
While blue is the most recognized color, raw sapphire occurs in an extensive spectrum of hues, including yellow, green, pink, purple, and colorless varieties. Trace elements like iron and titanium cause the famous blue color. The exception is red corundum, which is classified separately as ruby, despite sharing the same mineral structure.
The stone’s surface reveals its characteristic luster, which is typically described as vitreous (glass-like), but can appear adamantine (diamond-like) on clean crystal faces. A magnifying glass or loupe can reveal important details about transparency and color distribution. Raw corundum ranges from completely opaque to transparent, with transparent specimens being the most valued.
A common feature is color zoning, which appears as uneven patches or bands of color, reflecting fluctuations in trace element supply during growth. Furthermore, the presence of fine, needle-like inclusions of rutile, often called “silk,” can confirm a natural origin and may cause the optical phenomenon known as asterism in specific cuts.
The Essential Hardness Test
The single most definitive test for raw sapphire is the scratch test, which assesses its hardness on the Mohs scale. Sapphire is rated at 9, a level second only to diamond, making it exceptionally resistant to scratching. This high hardness means that true corundum should be able to scratch nearly every other common mineral it might be confused with, including quartz and topaz.
To perform this test practically, attempt to scratch the specimen with a material of known, lower hardness, such as a piece of quartz (Mohs 7) or a steel file (typically Mohs 6.5-7). Hold the specimen firmly and press the sharp point of the test material firmly across a clean surface of the suspected sapphire. After the attempt, wipe away any powdered residue left by the tester to see if a distinct, permanent groove has been etched into the surface.
If the quartz or steel leaves a scratch, the material is not sapphire; if the sapphire remains unmarked, its hardness suggests it is corundum. Because this process involves applying force and sharp points, always wear appropriate eye protection and secure the specimen in a stable holder. Testing the stone against a known piece of topaz (Mohs 8) is even more conclusive, as a true sapphire should easily scratch the topaz.
Common Look-Alikes and Misidentifications
The most common mineral mistaken for raw sapphire is quartz, largely because it is abundant and can occur in similar colors, particularly blue, yellow, and green. Quartz has a Mohs hardness of 7, meaning a true sapphire will easily scratch its surface, while quartz will not leave a mark on corundum.
Another common confusion is with topaz, rated at Mohs 8, but it is still softer than sapphire. Tanzanite, a violet-blue gemstone, can also resemble blue sapphire, but its hardness is much lower, ranging from 6 to 7 on the Mohs scale.
Manufactured glass fragments, occasionally found in stream gravels, are amorphous and have a hardness around 5.5, making them easily scratched by a steel tool. Differentiating based on crystal structure is also helpful, as glass is non-crystalline and will lack the distinct six-sided habit of corundum.