パール - 穴あき
Pearls are products of bivalve mollusks (mainly oysters and mussels). They are built up of nacre (mother-of-pearl), which is mainly calcium carbonate in the form of aragonite crystals, and an organic horn substance known as conchiolin that binds the microcrystals concentrically around an irritant. Although pearls are not especially hard, only about 2.5 to 4.5 on the Mohs scale, they are extraordinarily compact and very difficult to crush.
The typical pearly luster is produced by the overlapping platelets of aragonite and film of conchiolin nearest to the pearl surface. The color of pearl varies with the type of mollusk and the water, and is dependent on the color of the upper conchiolin layer. Pearls may be natural (occurring without human intervention) or cultured; and they may occur in the ocean or in freshwater.
It's pretty great having a whole lot of different gemstones to choose from. When doing crafts and jewelry making, it's great to have all kinds of options, and it's amazing how much cheaper it is to just purchase un-set gemstones and make the setting yourself, as opposed to buying fully-made jewelry already. It's also extra special since you made it yourself.