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There are two main types of these beads. The first type which is much the more usual has the pattern made with white lines on a background of the natural colour of the stone. The second type has the whole surface of the stone whitened and then a design in black made upon it, the whitened surface of the stone forming a white background. Specimens of this type are very uncommon.
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Horace C. Beck (1933) studied this question.
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