Reading a specimen.

Matrix

The rock a crystal grew on or within. A specimen "on matrix" is more desirable than one detached because the matrix provides geological context and confirms authenticity. Beware of specimens that have been glued back onto matrix — cheap specimens are often reattached, and the glue line is visible under loupe.

Termination

The end of a crystal where growth terminated. A "sharp termination" means the faces meet cleanly at a point. "Doubly terminated" means both ends are naturally formed (rather than broken from matrix). Sharp, undamaged terminations dramatically increase value.

Associated minerals

Other minerals on the same specimen. An aquamarine on muscovite is more valuable than an aquamarine alone because the association pins down the locality (Shigar, Pakistan, typically). Some associations are prized in their own right: fluorite with calcite, azurite with malachite, smoky quartz with feldspar.

Luster

How a crystal surface reflects light. The most common terms: adamantine (diamond-like, highest), vitreous (glassy — common for quartz), resinous (amber-like), pearly (layered surfaces), silky (fibrous minerals), greasy (dull but wet-looking), and earthy (matte, lowest).

Habit

The characteristic shape a mineral typically forms. Quartz is "prismatic" (elongated). Pyrite is "cubic". Garnet is "dodecahedral". A specimen that shows its species's textbook habit clearly is more valuable than one that is malformed.

Crystal system

The symmetry class of the crystal lattice. Seven systems in total: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, trigonal. You don't need to memorize these — but knowing which system your specimen belongs to is table stakes for catalog documentation.

Inclusions

Foreign material trapped inside the crystal during growth. Some inclusions are desirable (rutile needles in quartz, three-phase inclusions in emerald that prove locality). Others are flaws that reduce value. The distinction lies in whether the inclusion is diagnostic or merely disruptive.

Twinning

Two or more crystals of the same species intergrown in a symmetrical pattern. Some twinning is diagnostic (staurolite crosses, fluorite interpenetrant cubes). Twinned specimens are often rarer and priced accordingly.

Grade classifications

The specimen world uses informal tiers:

• Museum grade / A+ / exceptional: top 1% of its species and locality. Suitable for institutional display.
• Fine / A: excellent specimen, minor flaws only.
• Very good / B: well-formed, minor damage.
• Good / C: typical material, visible imperfections.
• Study / D: for educational or comparative use only.

Grade is a judgment, not a measurement. Trust the seller's reputation before you trust the label.