OPAL. 
GENERIC character. 
A hard, smooth, shining stone. 
Semitransparent. 
Fracture conchoidal. 
Infusible by itself, but melts in a strong heat when mixed 
with mineral alkali, or borax. 
SPECIFIC CHARACTER. 
Op alus vulgaris. Linn . Syst. Nat. Gmel. 3. p. I6l. 
Achates fere pellucida, colores pro situ spec- 
tatoris mutans. Waller. Miner. 82. 
An almost pellucid agate, whose colours 
change according to the position of the 
spectator. 
Silex vagus, reflectione et refractione varians j 
albus. 
Opal Cronst. Miner. 1 . p. 157. Kir wan, Miner.!, 
p. 289- Patrin, Hist, des Miner. 2, 
p, 2l6. Brogn. Miner. 1 . p. 500. 
The opal is much esteemed among naturalists for 
the delicacy of its colours. Buffon ranks it above 
all the sparkling gems ; for although it has neither 
the hardness nor brilliancy of precious stones, yet 
