Ruby is the birthstone for June. Ruby colours vary from pink to blood red due to the presence of chromium. Only red corundum can be called ruby, everything else is sapphire. Rubies of more than 3 carats are very rare due to the chrome which colours this gem, which causes fissures and cracks within ruby crystals. Rubies sometimes exhibit a silky shine or ‘silk’, which is caused by fine rutile needles; which in abundance creates asterism which we know as star ruby.