c blocks at a place are
of the same kind, and resemble in their nature the underlying or
neighbouring rocks.

It is this weathering process which has originated the gem sand of
Ceylon. Precious stones have been found disseminated in limited
numbers in the granite converted into _cabook_. In weathering, the
difficultly decomposable precious stones have not been attacked, or
attacked only to a limited extent. They have therefore retained
their original form and hardness. When in the course of thousands of
years streams of water have flowed over the layers of _cabook_,
their soft, already half-weathered constituents have been for the
most part changed into a fine mud, and as such washed away, while
the hard gems have only been inconsiderably rounded and little
diminished in size. The current of water therefore has not been able
to wash them far away from the place where they were originally
imbedded in the rock, and we now find them collected in the
gravel-bed, resting for the most part on the fundamental rock which
the stream has left behind, and which afterwards, when the water has
changed its course, has been again covered by new layers of mud,
clay, and sand. It is this gravel-bed which the natives call
_nellan_, and from which they chiefly get their treasures of
precious stones.

Of all the kinds of stones which are used as ornaments there are
both noble and common varieties, without there being any perceptible
difference in their chemical composition. The most skilful chemist
would thus have difficulty in finding in their chemical composition
the least difference between corundum and sapphire or ruby, between
common beryl and emerald, between the precious and the common topaz,
between the hyacinth and the common zircon, between precious and
common spinel; and every mineralogist knows that there are
innumerable intermediate stages between these minerals which are so
dissimilar though absolutely identical in composition. This gave the
old naturalists occasion to speak of ripe and unripe precious
stones. They said that in order to ripen precious stones the heat of
the south was required. This transference of well-known
circumstances from the vegetable to the mineral kingdom is certainly
without justification. It points however to a remarkable and
hitherto unexplained circumstance, namely, that the occurrence of
precious stones is, with few exceptions, confined to southern
regions[390]. Diamonds are found in notewort