Identifying Inclusions of Specific Gems

Step 2: Advanced Gemology

Lesson 12

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Beryl and Tourmaline

Both beryl and tourmaline develop hollow or “growth” tubes. Gem cutters see many more of these than gemologists do, since they cut away most of these inclusions. While these tubes begin hollow, they may be filled with opaque, white minerals later, as the crystal develops.

You can easily distinguish growth tubes from other inclusions. They have even sides and run straight for considerable distances. When present, you can assume these inclusions indicate a gem is most likely a beryl, tourmaline, or possibly spodumene. A refractive index (RI) reading can then distinguish these minerals.

Corundum

Rubies and sapphires, both corundum gemstones, are some of the easiest gems to identify by their inclusions.

Silk in Hexagonal Patterns

Silk is the most distinctive of corundum’s inclusions. What gemologists call “silk” consists of very fine threads (crystals) of another mineral, rutile. If you can see enough of these inclusions, you’ll note they form a hexagonal pattern. If you can only see part of the threads, they meet at 60° or 120° angles.

Garnet gems contain similar but coarser inclusions. They meet at 70° or 110°. On the other hand, …