The Aquanauts mined and collected the crystals for survival. They used submarines and other vehicles to collect the crystals (and fight off opposing Aquasharks) and bring them back to their base, the Neptune Discovery Lab.
For the Rock Raiders, based on the video game and promotional materials, the crystals represented the only source of power for the stranded crew. The crew crash landed on the mysterious planet, and they had to collect crystals to power and repair their vehicles to eventually make their escape.
In more recent Town, Lone Ranger, and Superheroes sets the pearl gold and silver crystal appears as a source of valuable wealth, basically gold.