Glass

Glass was a clear substance. Sand could be made into glass, and would eventually do so if left alone for long enough. (TV: "The Sea of Death", The Caves of Androzani) Broken, it could cut deep through flesh; Lady Casaubon stabbed Orlo with a piece of glass, and then shot him with a glass bullet. (PROSE: Martha in the Mirror)

The term "glass" was also used to describe cups made of glass. The First Doctor once remarked that Marinus had "beautiful glasses". (TV: "The Velvet Web") The Eleventh Doctor also used a glass cup to listen to the other side of the crack in Amelia Pond's bedroom. (TV: The Eleventh Hour) "Glasses" also referred to spectacles, which had glass lenses. (TV: The Girl Who Waited, The Angels Take Manhattan, et al.)

Manfred Grieg, who had been trapped in the Mortal Mirror, was cursed with a perpetual body of glass, and kept a glass diary. (PROSE: Martha in the Mirror)

A Wanarian battle cruiser was capable of turning an area the size of Cardiff into glass. (PROSE: Pack Animals)

Ian Chesterton once hypothesised that a crystal box was made of glass, although the First Doctor quickly dismissed his idea, saying that it must have been "more sophisticated than that." (TV: "World's End")

Physical properties
Glass was described by Ian Chesterton as "hard," (TV: "The Executioners") and by Turlough as "smooth." Norna stated that you could "see through glass." (TV: Frontios) One's reflection could be seen on glass, as well. (TV: Day of the Moon, et al.) The Fifth Doctor once described crystalline material as "polished smooth as glass." (TV: The Caves of Androzani) In order to prove to Grace Holloway that the Eye of Harmony was harming the molecular structure of Earth, the Eighth Doctor walked right through a glass door. (TV: Doctor Who)

Uses
Due to glass' sharpness, a beach on Marinus was lined with broken glass as a defence mechanism. (TV: "The Sea of Death")

Some buildings, such as the Shard (TV: The Bell of Saint John) and the Gherkin, (TV: The Christmas Invasion) were made almost entirely of glass. Clockfaces were made of glass. (TV: The Girl in the Fireplace) At least one of the components of a reticular vector gauge was also made from glass. Mirrors were also made of glass. (PROSE: Martha in the Mirror, TV: The Family of Blood) The Exhalation was an entire planet made of glass. (PROSE: Heart of Glass)

Art was also made from glass; stained glass windows were often found in places such as churches. (TV: The End of Time) It was also popular in rich households, at least during the Victorian era. (TV: Ghost Light)

Barbara Wright once claimed that people put "freaks" in glass cases and examined them with microscopes. (TV: "Dangerous Journey")

Glass windows were found at the front of shops so that the merchandise could be seen clearly from outside. Mannequins sometimes stood behind those shop windows. (TV: Rose) In the 21st century, some shops had automated glass doors, that "know you're there". As Ianto Jones began to explain, "there are wave-bouncing detectors which emit radio waves and then look for reflections." (TV: Out of Time)

A magnifying glass was a glass that produced a magnified image of an object. Sun rays sent through a magnifying glass could be used to burn things. (TV: The Power of the Daleks, The Unicorn and the Wasp, AUDIO: The Spectre of Lanyon Moor)

In the 19th century, common illusions were made using "projection on glass", according to Charles Dickens. (TV: The Unquiet Dead)

Durability
Finitoglass was a type of glass known for its durability, and could stand the radiation of an X-tonic ray for several minutes. (TV: Midnight) Vinvocci glass was capable of containing at least 500,000 rads of radiation. (TV: The End of Time) The Fourth Doctor once used high-pitched screaming to shatter glass; he claimed that he picked the technique up from Nellie Melba. (TV: The Power of Kroll)

Glass in general was considered durable, and was used to protect valuables. Some form of glass case was used to protect the Mona Lisa in the Louvre, although a sonic knife was able to cut through the glass. (TV: City of Death) A bank vault was made with titanium and protected by laser-proof glass and destructor rays. (COMIC: Time of the Cybermen) Glass was also used to trap people; Bernice Summerfield was once trapped in a glass prison by the Fifth Axis. (PROSE: The Glass Prison) Pinehill Crest Hotel's doors, made of safety glass, could not be broken even with a fire extinguisher. (AUDIO: The Shadow of the Scourge)

Despite glass's durability, when the Tenth Doctor fell through a glass ceiling, it shattered all around him. (TV: The End of Time) Anything heavy or strong, such as a Minotaur, could shatter glass. (TV: The God Complex) The Tenth Doctor once demanded a truck in order to break through glass, although he eventually did so with a horse instead. (TV: The Girl in the Fireplace) Judoon blasters could shoot through glass without difficulty. (TV: Smith and Jones) Davros once claimed that "the tiny pressure on [his] thumb [would be] enough to break the glass." (TV: Genesis of the Daleks)

Extreme sonic wavelengths, such as those caused by two identical sonic devices being held together, such as the Tenth Doctor's sonic screwdriver and Miss Foster's sonic pen, could shatter glass. (TV: Partners in Crime) Sometimes, this could happen simply using one device, such as the sonic lipstick. (TV: The Day of the Clown) When a spaceship hit Earth's atmosphere, the glass in at least central London shattered. (TV: The Christmas Invasion) As a result of this, Elton Pope could not move because of the sharp glass on the floor. (TV: Love & Monsters)