Obsidian and rock crystal were also used for knife making.
The intention would seem to have been to imitate vessels of rock crystal.
They are made usually of crown glass or rock crystal ("pebbles"), the latter being somewhat lighter and cooler to wear.
He fell upon a most ingenious plan of doubling the amount of double refraction of a prism by using two prisms of rock-crystal, so cut out of the solid as to give each the same quantity of double refraction, and yet to double the quantity in the effect produced.
There appears, however, to be some objection to the use of iridio-platinum for weights, as, owing to its great density (Δ=21.57), the slightest abrasion will make an appreciable difference in a weight; sometimes, therefore, quartz or rock-crystal is used; but to this also there is some objection, as owing to its low density (Δ=2.65) there is a large exposed surface of the mass.
