Stones is the industry name for nickel sulphide inclusions in glass that may crystallize and grow with time and create stresses within the glass that may shatter the glass only after months or years of use.

One of the chief causes of spontaneous shattering.

Other inclusions are fragments of refractory brick from the walls of the glass smelting kiln.

Spontaneous breakage caused by nickel-sulphide stones occurs only in tempered glass, not in annealed or heat-strengthened glass.



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Note added at 9 hrs (2013-12-09 21:44:31 GMT)
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This Q&A document explains how nickel sulphide inclusions (aka stones) can shatter glass unexpectedly.

http://tinyurl.com/ntvpppm

Inherent in the glass production process are microscopic imperfections in the glass, known as inclusions. Most of these are completely harmless, but nickel sulfide (NiS) inclusions have been shown to cause disastrous failure of tempered glass. When annealed (aka float) glass is heated in the tempering process, so are any NiS inclusions present in the glass. However, when the glass is rapidly cooled to achieve the properties of tempered glass, the NiS remains in a high-temperature form. Over several years, the NiS will return to its low-temperature state, and in the process will increase in volume. This can cause cracking and additional tensile stresses which, in tempered glass, have lead to spectacular failures with no visible cause. This phenomenon has also been referred to as “glass cancer” and “spontaneous glass failure”.

"Major strides have been made in reducing the contamination of raw materials, and great care is taken to avoid the contact between the raw materials and any nickel-containing alloys (such as stainless steel). In addition, all glass furnaces now use natural gas instead of fuel oil, because it was found that fuel oil contains nickel oxide (NiO) in trace amounts up to 3 parts per million (ppm) (Kasper and Stadelmann 2002). This may seem insignificant, until it is considered that this quantity of nickel is sufficient to form a nickel sulfide inclusion of a potentially dangerous size every 2mm on a float line (Kasper and Stadelmann 2002). In fact, one gram of nickel, once made into NiS, has the potential to contaminate all glass produced on a typical float line for ten days (approximately 6000 tons of glass) with NiS stones (Kasper and Stadelmann 2002). It is obvious with such small concentrations being so incredibly problematic, any nickel that enters the float glass can become a serious concern."
Source: http://failures.wikispaces.com/Glass Breakage - Nickel Sulfi...

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Note added at 9 hrs (2013-12-09 21:50:44 GMT)
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Glass trivia: on a visit to the Pilkington's float glass works in St Helens, UK, we were all issued with clogs - wooden soled industrial footwear.

On the float line, a bath full of molten tin on which the sheet glass is made, the floor temperatures are so high that other sole materials simply can't offer sufficient insulation.

This industry preserved the ancient craft of clogmaking in the UK.

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Note added at 9 hrs (2013-12-09 21:54:38 GMT)
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This paper is quoted in several references:

J .C. Barry (1993) “A study of nickel sulphide stones in tempered glass”, ... microcracking induced by a volumetric expanding phase change”.