Crystalline carbon and the hardest substance known to man.  Some call it a girl's best friend.  I call it April's birthstone.

Hardness:  10
Toughness:  good in cleavage directions, exceptional in all others
Color:  Usually light yellow, brown, or grey.  Also occurs in every color of the rainbow.
Transparent to opaque.  Usually eye clean and commonly available in large sizes

Care:
Ultrasonic:  usually safe, unless feathers are present
Steam:  usually safe, unless feathers are present
Warm, soapy water:  safe

Stability:
Heat:  begins to vaporize in an oxygen rich atmosphere at 690-875°C
Light:  stable
Chemicals:  stable

Major sources:
Botswana, Russia, South Africa, Australia

Treatments:
Many treatments available to improve the beauty of less marketable stones: irradiation, coating, fracture filling, laser drilling and bleaching, HPHT, etc.  These treatments and their special care requirements should always be disclosed.