This is also known as species in some circles.
This heading was a difficult one to decide on, because it could so easily have come under the heading gem groups, or gem species. The whole topic of which gem is related to which family or which group, or which species, is complicated by the fact that various organisations in the gem world use different terminology for different connections.
Let me try and make it as simple as possible by looking at the GIAs description found in their publication of the “Gem Reference Guide” in 1988. Even though it was written 23 years ago, I still feel it is the most comprehensive and accurate guide ever written on the technical aspects of gemstones and with regards to this subject, they are very clear on their views.
Firstly the GIA teach us that there are only two “Groups” of gemstones and these are the Feldspar and Garnet Groups. I personally add a third group to this and treat Organic gemstones as a group name.
All other parents such as Quartz, Tourmalines and Beryls are know by the GIA as “species”. However, today it is more common to hear these referred to as “families”. So don’t get confused if you hear either term, they are simply referring to the same thing.
Over the page are a few of the species/family names are shown along with their main family members. So let us look at two examples which will hopefully make it easier to understand:
(1) Tsavorite Garnet is the variety name of a rare Green member of the Grossularite Garnet species/family, which is in turn part of the Garnet group.
(2) To slightly complicate this three-tier system, in some families/species, there are separate branches/sub species (these are not industry terms but are the best names my team and I could come up with). For example the Quartz family/species can then be further divided into Cryptocrystalline (whose crystals are microscopic like in Chalcedony and Agate) and Crystalline sub species, such as Amethyst, Citrine, Smoky Quartz, Strawberry Quartz and Rose Quartz. But what is it that relates certain gems to others? Well, that’s unfortunately not quite as straightforward! The reason why some gems are related to others depends on which families you are talking about and most families are grouped for different reasons. All members of the Quartz species/family are related due to their chemical composition being the same (SiO2). All Beryls also share the same chemical composition Be3AI2Si6O18. Tourmalines on the other hand, all share the same crystal structure, but vary in chemical composition.
