White spice is an extremely rare spice that has been found mostly on colonized gas giants, although there have been reports of it being found on normal planets as well. It is impossible to get this spice via colonizing, since Spore does not let you place a colony on any gas giants. However, if the game has placed a colony on the gas giant for an NPC empire, you can buy the system from them as normal, thus obtaining the gas giant (but they are T-0, and stop producing it). White spice can be collected like any other spice, but cannot be bought or sold. Finding white spice naturally on a gas giant is extremely rare, but mods can be used to obtain it. In the file "Spore_game.package", there are files that have the percent chance that a certain color of spice will appear. If the percentage for one kind of spice is lowered and the empty space is not filled in, the result will be white spice. Therefore, white spice is a placeholder in the event that a planet has no spice assigned to it. These percentages apparently don't apply to gas giants, as white spice is the only kind found on them since they were not meant to be colonized in the first place. The White spice icon is actually the neutral image for all the spice icons, and the game recolors the base image to display different spices.
White spice on planets:
Sometimes white spice can be found on colonizable planets. Colonies of your species will be treated as another empire, but you still get the 'planet harvest' and you can sell your spice to your own colony. To be clear: it will still be a colony and planet of your empire. You can sell any spice to it for the price it would sell to another empire (except white spice, being untradeable). When the spice is harvested or collected from the colony, its name in the cargo hold is just 3 small stars, and the icon is the Spore weapons icon. So far there is no way to trade/sell it. It can be jettisoned.
It is possible that this results from a floating-point rounding error when attempting to produce a weighted random number. If that is correct, the occurrence chance is somewhere on the order of 2^-40. If anyone has any further details on this, it would be greatly accepted.
White Spice can also be confused with tornadoes, because if you find them in water they make "holes" in the water.