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    It's very common for any minerals to grow first in the cavities before agate. Calcite, geothite, chlorides, and zeolite minerals are the examples that tend to be first to grow in the cavities before being covered with the agates and it is common for the minerals to be replaced by agate. But there are times when some minerals are preserved and isolated by the agate.
    "Sagenite" is sort of a nickname given to the radiating needle-like structures within the agate nodules (see pic above) and you can see that they tend to look like balls of needles.

Aragonite crystals coated with agate layer.
Needle Peak, Texas

Another example of "Sagenite" agate.

An exampe of very brushy and very reddish ""Plume" agate from Mexicio

A black calcite grew on the agate layers below the white layer before being covered with additional agate layers.