I think it’s Boulder opal has a wave like pattern which is unique.
eBay
Necklace Fashion Natural Black Opal Pendant with Necklaces for Women Elegant Charming Opal Christmas Jewelry Gift
Aliexpress UK
This is a brand new Nadean Designs, an Etsy artisan, black opal necklace on a gold filled chain. The necklace is 17 inches long and the pendant is 0.6 inches tall and 0.3 inches wide. It weighs 2 grams. Has a tag saying 'Nadean Designs'.
eBay
TONGZHE Opal Heart Necklace Multicolor Synthetic Opal 925 Sterling Silver Jewelry Choker Love Necklace Pendants Women&30
Aliexpress UK
Female Small Pendants Necklaces Rose Gold Wedding Necklaces for Women Black Natural Opal Necklace
Aliexpress UK
Aubao Necklace Vintage Dopamine Necklace Opal Pendant Micro Set Zircon Copper Plating Mother's Day Necklace
Aliexpress UK
New Natural malachite agate opal 4mm round bead necklace Men Women Beach Necklaces Energy Healing Reiki Necklace Jewelry Gifts
Aliexpress UK
IPARAM Fashion Trend Crystals Necklace Bohemian Hexagon Opal Pendant Necklace Female Hexagon Crystal Necklace Gift NEW
Aliexpress UK
New Fashion Handmade Opal Jewelry 925 Sterling Silver Opal Horse Pendant Necklace Jewelry For Gif New
Aliexpress UK
Water droplet pendant necklace, stainless steel inlaid with imitation opal, opal pendant, sweater chain, gold-plated
Aliexpress UK