Pebble lenses may be distinguished from common glass spectacle lenses in the following manner. If the tip of the tongue be placed on a piece of glass it will feel rather warm and smooth, or woolly; but if the tongue be placed on a piece of quartz it will be cold, with a peculiar crisp feeling. Another test is hardness; a crystal of quartz will readily scratch glass, but the crystal will run over a pebble without leaving any scratch. A natural stone is a much better conductor of heat than any glass, and so to the tongue will feel cold; and being a variety of quartz, it will not be scratched by another crystal of quartz. If the pebble is supposed to be, say, a topaz or a ruby, then, being harder than quartz, it will in its turn scratch quartz. If the pebble is a diamond, then it will scratch a ruby or sapphire. Another rough and ready method of testing hardness is to pass a small fine-cut file over the edge of a bit of glass; there will be a somewhat dull, cutting sound emitted. If the file be passed over a bit of quartz the sound will be clearer and sharper.