Habit, form and termination
Updated
Two specimens of the same species from the same mine can differ in price by an order of magnitude, and habit is most of why.
What habit means
The characteristic shape a crystal takes: cubic, octahedral, prismatic, tabular, acicular, botryoidal, massive.
It is a property of how the crystal grew rather than of what it is, so one species shows several habits and the habit is often diagnostic of the locality.
Why it decides value
A sharp, well-formed crystal is rarer than a malformed one from the same pocket, and collectors pay for form more than for size.
An unusual habit for a species is collectible in its own right, sometimes more than a better example of the usual one.
And aesthetics: a single sharp crystal well-positioned on matrix outsells a cluster of equal total weight, reliably.
Termination
Whether the crystal ends in natural faces or in a break.
A terminated crystal is complete as it grew. A broken one is an object that has lost part of itself, and the difference in price is large.
Doubly terminated, complete at both ends, having grown free rather than attached, is rarer again and priced accordingly.
Which is why the fragile end is the part to protect in packing, and why lifting a specimen by a crystal is how value gets destroyed.
Describing it
Habit, then size of the main crystal, then whether terminated, then matrix, then associated species.
Describe damage plainly. "Minor contact on one edge" is a normal, expected part of an honest description, and a buyer who finds undisclosed damage remembers it longer than they remember a discount.