22 Preface يَخرُجُ مِنهُمَا اللُّؤلُؤُ وَالمَرجانُYusuf AliOut of them come Pearls and Coral:1Pearls are produced by the oyster and coral by the polyp, a minute marine creature which, working in millions, has by its secretions produced the reefs, islands, and banks in and on both sides of the Red Sea and in other parts of the world. The pearl has a translucent lustre, usually white, but sometimes pink or black. Coral is usually opaque, red or pink, but often white, and is seen in beautiful branching or cup-like shapes as visitors to Port Sudan will recall. Both are used as gems and stand here for gems generally. Mineral gems, such as agate and cornelian, are found in river-beds. Pearl oysters are also found in some rivers. Taken allegorically, the two kinds of gems would denote the jewels of this life and the jewels of the spiritual world. The jewels of this world?like coral?are hard, widely spread over the world, comparatively cheap, and less absorptive of the light from above. The spiritual jewels?like pearls?are soft, rare, costly, and translucent, absorbent of light and showing the more lustre the more they are in light. The analogy can be carried further to the two seas?the two bodies of flowing water? mentioned in verses 19-20 above. They are the two kinds of knowledge?human and divine?referred to in the story of Moses and Khidr: see notes 2404-2405 to 18:60.