Recently black pearls have been successfully cultivated in the Ishigaki Islands, Okinawa, and also in Tahiti Islands. Blue pearls have also been obtained occassionally among ordinary cultivated pearls. These have already appeared in the market. On the other hand, artificial colouring of pearls to obtain black and blue pearls has been performed and the products have been in the market since 1930. It has therefore become an urgent need to establish non-destructive methods to differentiate between true and artificially coloured black and blue pearls. To achieve this purpose, both true and artificially coloured black and blue pearls are investigated by means of (1) observation of fluorescence under ultraviolet light, (2) observaton under the polarization microscope, (3) investigation of different colouring on infrared film, (4) X-ray fluorescence analysis (black pearls), and (5) fiberscopic observation (blue pearls). It was found that true and artificially coloured black pearls may be easily distinguished by either a combination of four different methods or simply by comparing different colouringi on infrared films, but in the case of blue pearls, further studies are necessary. The definition of black and blue colors, origin of the colours, the methods of cultivation of black pearls are also described in some details.

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台湾猫眼石はトレモライトキャッツアイであり, その宝石原料のトレモライトは花蓮県寿豊郷豊岡村付近の石墨絹雲母片岩と蛇紋岩シルとの接触部に小派をなして産し,ネフライト,石綿,滑石,透輝石を主とするスカルンなどと共生する(図1,2,表1). 独眼石は緑黄,淡黄,蜂蜜色,緑,まれに暗緑, 暗褐ないし黒色を呈し, 半透明ないし不透明である. 硬度6〜7, 比重平均3. 045. 鏡下では無色ないし淡緑色で, すべて細長いほぼ平行なトレモライトの繊維状結晶集合からなり,いわゆるネフライトの構造がない. 繊維の捻曲や波状消光が往々見られる. 消光角は0゜〜10゜, 浸液法による主要屈折率はα 1.613, β 1.626, γ 1.637, 複屈折は0.024である. 粉末X線資料はスイスセイントゴッタード産のトレモライトに類似している(表2). 四つの違った色のキャッツアイのAA分析の結果は著しい化学成分上の差異はなく,いずれも約10%の鉄陽起石分子を含む(表3). また,ネフライトともほとんど同じである(表4). 普通のカボションカットでは,長軸を繊維に直角させ, 底部を片理に平行に作ると, シャトヤンシーが短軸, すなわち繊維の方向に直角に現われ, それに沿って動く(図3). ただし曲った繊維の部分はシャトヤンシーの湾曲を来たし, また繊維の離れ易いものは宝石原料には使われない. 猫眼石はネフライトに比べて原石が非常に少ないので,やや高価である. 透輝石は以前報告したよりも多くの結晶形を示し, その化学成分と光学的性質を表5に示す. グロシュラー柘榴石の産出は接触部近くの透輝石グロシュラー柘榴石脈に限られている. スカルン鉱物はこれらのほかに少量の緑簾石グループの鉱物があり, また, 台湾で始めてのクロム柘榴石とヴェスビァナイトが発見された. 前者は透輝石中の微小包有物として産し, 屈折率1.778, α_0 11.87A^^○で 10% Cr_2O_3を含むものであり(表6), 後者は透輝石グロシュラー柘榴石脈中の小柱状結晶で X線資料はイタリア, カンソコリ産のヴェスビアナイトに似ている(表7). 豊田ネフライト石綿鉱床の鉱化作用はスカルン生成と鉱石沈殿の2期に分けられ,いずれも蛇紋岩化作用の間に母岩から珪酸と石灰を多量溶かした熱水溶液の作用によるものと思われる. キャッツアイトレモライトは明らかにネフライトよりも後期の生成を示し, 鉱床生成後も引続き動力変質を受けた証拠がある.

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Japanese pearl industry who had enjoyed its rapid growth and boom after the close of the world War II suffered from severe depression starting from 1966, and by 1970 more than 550 companies both in fishery and trade in pearl industry went bankrupt. The Japanese pearl industry and trade were deeply depressed, but this depression was limited only in the field of pearl industry and trade, other fields had maintained the booming. The main cause of the depression is refered to that foreign buyers of pearl sharply cut down the amount of pearl to purchase, because they had at the time over stock-in-trade, and this naturally resulted in sharp decrease in price of Japanese cultured pearl. The price went down by 30% in 1967 as compared to the price of the previous year. This low price continued until 1970. The initiative in settling the price of cultured pearl which was on the side of Japanese producers now transfered to the hand of foreign buyers. The depression was in the worst in 1971, when the price per momme was only 824 Japanese yen. During 1966 to 1971, all efforts have been made to reduce the production to a half, and to control the price by means of stocking the over-production by Governmental agencies. From 1972 to 1975, the situation gradually turned to a better, and in 1975 the depression was successfully overcome. The price in 1975 recovered to 2247 Japanese yen per momme.

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