This specimen is an example of a placer mineral that formed in a sedimentary environment.
Sedimentary: Placer Minerals
Minerals, especially those heavier than average, may be mechanically separated and concentrated as they are moved from one place to another by flowing water or wind. A concentrated surface deposit of such minerals, known as a placer deposit, forms in two successive stages.
In the first stage, weathering frees the minerals from the original rock material. In the second stage, the minerals are mechanically carried, separated by gravitation and concentrated elsewhere, usually along a streambed or on a beach. Concentration into a placer deposit can occur only if the minerals are heavy (i.e., possess a high specific gravity), are chemically resistant to weathering and durable.
Placer deposits such as those displayed here commonly consist of gold, platinum, cassiterite, magnetite, chromite, zircon, and gemstones — ruby, sapphire, and garnet, for example.
Wataroa, New Zealand