Crystal Beetle Swarm (CR 6)

Medium Vermin (Earth)
Alignment: Always neutral
Initiative: +0; Senses: tremorsense

AC: 19 (+9 natural), touch 10, flat-footed 19
Hit Dice: 9d8+27 (67 hp)
Fort +9, Ref +3, Will +3
Speed: 30 ft., burrow 30 ft.
Space: 5 ft./0
Base Attack +6; Grapple -
Attack: -
Full Attack: -
Damage: Swarm 2d6 plus 2d6 acid
Special Attacks/Actions: Encase, distraction (DC 16)
Abilities: Str 1, Dex 10, Con 16, Int --, Wis 10, Cha 2
Special Qualities: Acid immunity, Swarm traits, vermin traits
Feats: -
Skills: -
Advancement: --
Climate/Terrain: Any land and underground
Organization: Solitary or army (10-40 swarms)
Treasure: None

Source: Web

Crystal beetles are ubiquitous fist-sized vermin that primarily dwell on the Plane of Earth. They fulfill a similar ecological niche as many other burrowing insects do on the Material Plane -- churning through rock and turning it into rich soil. Their name derives from their crystal-like carapace, which shines and scintillates on the rare occasions when light shines upon them. It is transparent, allowing the viewer to see the organs inside the beetle's body. The shell of a crystal beetle is made from a hard, transparent substance that resembles a diamond or rock crystal. Some swarms find their way to the Material Plane, where they will happily feast on veins of ore and gems, as well as any hapless creatures that blunder into their path. Certain creatures native to the Plane of Earth, such as xorns and umber hulks, find crystal beetles delicious (if dangerous) food and gorge on swarms when they find them.

Crystal beetles feed upon stone, earth, and gemstones. To help them digest such tough matter, they exude a crystalline substance that releases a powerful acid. In a matter of minutes, the stone is reduced to a more palatable goo that they feast upon. Crystal beetles apparently have a "sweet tooth" when it comes to living creatures -- a swarm of crystal beetles will overrun a creature, encase it in crystal, and feed upon the remains. Such a rare treat will draw nearby swarms, which will completely cover the victim in hope of gaining a taste. The sound of a crystal beetle swarm burrowing through the stone sounds like a rush of water.

Despite its resemblance to gemstone, the crystal beetle has no value other than as a curiosity. If a lone beetle becomes separated from the rest of the swarm, it dies in 1d6 days and slowly disintegrate from its own internal acid (causing no damage).

Earth Subtype

This subtype usually is used for elementals and outsiders with a connection to the Elemental Plane of Earth. Earth creatures usually have burrow speeds, and most earth creatures can burrow through solid rock.