Glass has an amorphous structure, which means that it components are arranged irregularly. When it breaks, uneven fragments with shell-like are produced. This is different to a crystal that generally breaks along even surfaces.
In nature, volcanic glasses (obsidian) can be found that consist of amorphous quartz and have been preserved in a chemically unaltered state for millions of years.
At Senzeilles in Belgium, tiny (0.05-1 mm) glass pearls produced by a meteorite impact 367 million years ago were found in a claystone. Because of their size, it could have been expected that they would have dissolved after such a long time. However, because they were enclosed in the clay, the glass pearls had not undergone any alteration.
Because of its favourable properties, glass forms the innermost engineered barrier in a geological repository for high-level waste. Radioactive elements are immobilised in a glass matrix which is very difficult to corrode or dissolve. The radioactive substances are thus safely contained for long time periods.