What are the basic things I need to consider when making such an important purchase?

Weight : Dependent on size preference and cost concerns, the number of carats or fractions there of will dictate the weight of the stone you choose.

Clarity : How clear is the stone when viewed by the naked eye or by aid of magnification? is it cloudy or clear in appearance?

Color : Still another factor to consider is the color of the stone. Less expensive gems tend to have more of a yellowish hue, while more expensive pieces will obviously approach a more brilliant or translucent appearance.