Chalcedony. 1 7S The green and blue varieties are the rareil. White and brown ftripes often alternate with each •ther, and form the variety named onyx. The milk white variety is known by the name cachalong. ^ The grey varieties with thick prifmatic diftinft con cretions, when tranfverfely cut, and held between the e ye and the light, exhibit rainbow colours, and hence have been named rainbow chalcedony. When it is cut parallel to the di.ftin£t concretions, it exhibits a clouded delineation. External fhape very various; befides maffive, in hlunt edged pieces, grains and rolled pieces, it occurs 111 original round balls (which are found in Amyg daloid, and often filled with water) reniform, botroi- dal, coralloidal, ftala&itical j alfo cryftallized in cubes ? Alfo found in extraneous external fhapes, as in. turbinites, tubulites, he. Internally almoft always dull; fome rare varieties exhibit a very faint degree of lull re. FraCture perfectly even ; it pafles fometimes, how ever, into fine fplintery, alfo into the uneven, which a Pproaches to the imperfectly flat conchoidal. The latter variety has a flight degree of luftre. Fragments indeterminately angular, and pretty fliarp edged. Generally occurs in reniform and fortification wife curved lamellar, ufually concentric, diftinCt concre- hons, which are commonly very thin, and have a r °ugh and dull furface. Commonly
