Topaz (aluminum fluorite silicate) is the hardest of the silicate minerals. It occurs naturally in yellow (most common), brown, clear, blue and red (rare). Blue topaz occurs when the clear form is exposed to radiation - while this occurs naturally, it is also a very easy process to duplicate in the lab and consequently, most blue topaz on the market today is created. The type of radiation applied dictates the resulting color - three hues have become standard: Sky Blue (lightest), Swiss Blue, and London Blue (darkest) with London Blue generally being the most valuable. With a hardness of 8 and a high refractive index, it is a lovely gem for faceting and very wearable. Care: generally trouble free; avoid hard knocks and ultrasonic or steam cleaning. Note: inexpensive light blue beads labelled "topaz" may in fact be dyed quartz - color fastness can vary considerably, depending on the source. I make an effort to avoid these.
The "water droplet" in the above pendant is a lovely little briolette of London Blue topaz.