All of your questions have been answered in the Meru Tantram, an important Agama Text.
First of all you need to understand that there are basically two kinds of Shiva lingam,1)Krittim Lingam(unnatural ,man made) & 2) Akrittim Lingam (naturally manifested).
Few examples of the Akrittim Lingams are the Swaymabhu Lingam(refer to the Jyotirlingams),Banalingam,Shiva Nabhi Lingam,Roudra Lingam etc.
Whereas, the examples of Krittim lingas are Sphatika lingas,Parada Lingas and a whole lot of other Lingas that are made of metals, stones etc.
The Meru Tantram text gives the relative importance of these Lingas in the following verses:
Komaleshu tu Lingeshu Parthivam Sresthamuchyate|
Kathineshu Tu PAshAnAth Sphatikam Param| Sphatikhath PadmarAgancha Kasmiram PadmaRAgatah| KashmirAth PushparAgotthamindranilodhbhavam
Tatah| IndranilAccha Gomedam GomedAdvidrumodbhavam| VidrumAnmouktikam
Sreshtham HairanyAdhvirakam Varam|
HirakAth PAradm Sreshtham Banalingam Tatah Param||
Among the Lingas made of soft materials Parthivalingas are the
best(those made of clay) & among those made of hard materials
Prastaralingas are the best(stone-made).Additionally,better than
Prastaralingas are Sphatika Lingas,better than Spahtika Lingas are the
Padmaragmani made ones,still better are the Kashmira Lingas,still
better are the Pushparaga made ones,still better are the Indranilamani
made ones,still better are the Gomeda made ones,still better are the
Vidrumani made ones,still better are the ones made of
pearls(mouktika),still better are the silver made ones,still better are
the gold made ones,better than the gold made Lingas are the Parada
Lingas & better than Parada Lingas are the Banalingas.
Another verse which depicts the greatness of Banalingas and Parada Lingas is :
SamsthApya SriBanalingam RatnaKotigunam Bhaveth Rasalinge Tato BAnAth Phalam Kotigunam Smritam||
What merit is achieved by worshiping crores of ratna(precious stones)
made lingas is obtained by simply by worshiping one Banalinga.And the merit that is obtained on worshiping crores of Banalingas is
obtained by simply worshiping one Parada Linga.
Now,it is interesting to note that, although Banalingas are Akrittim, Lord Shiva is giving more importance to the Parada Lingas even though they are Krittim.And the reason is simply that since Parada is considered as Shiva Veerya(the semen of Shiva) ,that's why its so auspicious.
So,among Krittim Lingas Parada Linga is without any doubt the best one.Way better than the Sphatika Lingas.And, Lord Shiva has even given more importance to it over the Banalingas