Not all slates are the same. As with most things in life, if you buy the cheaper options it will usually cost you more money later when you need to replace it. Slate roofing is no different. Slate that is cut from the cheaper, less pure, sources can be found to contain impurities and especially iron pyrites.
Iron pyrites will show as a rust coloured imperfection and will lead to rust stains on the roof. After many winters and frost, the slate will develop holes and can delaminate much quicker than better quality slates. Examples can be seen in the photos shown here.
This problem and inherent defect can be avoided by buying the best quality roofing slate. Slate that originates from England and Wales are usually of a high quality.
Imported slates from Spain, China, Brazil and Canada are usually of inferior quality and tend to fail sooner.