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Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly with limonite, sandstone, rhyolite, marl, and basalt. Opal is the birthstone of October. It is also the stone that celebrates the 14th anniversary of marriage. The name opal is believed to be derived from the Sanskrit word "upala", which means "jewel", and later the Greek derivative "Opallios", which means "to see a change in color". There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not.[4] Play-of-color is defined as "a pseudo chromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light."[5] The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent, or opaque and the background color may be white, black, or nearly any color of the visual spectrum.

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Prison education is any educational activity that occurs inside prison. Courses can include basic literacy programs, secondary school equivalency programs, vocational education, and tertiary education. Other activities such as rehabilitation programs, physical education, and arts and crafts programs may also be considered a form of prison education. Programs are typically provided, managed, and funded by the prison system, though inmates may be required to pay for distance education programs. The history of and current practices in prison education vary greatly among countries.Those entering prison systems worldwide have, on average, lower levels of education than the general population. Prison education often aims to make the inmate more employable after release. Administrating and attending educational programs in prisons can be difficult. Staff and budget shortages, a lack of educational resources and computers, and the transfer of prisoners between facilities are common barriers. Prisoners may be reluctant to participate, often due to past educational failures or a lack of motivation. Studies consistently show that education in prison is an effective way of reducing the rates of recidivism, which saves the expense of future prison sentences.

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