Nothing is credited with the magical power of divination more that the humble Crystal Ball. It has been mentioned in the book of revelations and even referenced in the legend of King Arthur.
Prominent in the Middle Ages, Scryers (Crystal Ball diviners) revered their crystals and often passed them on to their apprentices.
Crystal Balls are very tricky to make. The ball must be optically flawless and any that are not, get discarded. The larger the ball, the more difficult it becomes to make flawlessly and hence the larger sizes are disproportionately more expensive - but are very desirable objects.